The code tells you which safety tripped. It does not tell you why. Finding out why is the whole job.
Quick Answer
A tankless error code reports which safety limit tripped, not what caused it. Frisco Tankless Water Heater Pros troubleshoots recurring codes by reading stored fault history, measuring inlet and manifold gas pressure under full fire, checking flame rod signal in microamps, and testing flow and temperature rise. That distinguishes a gas supply problem from a failed part. Call (972) 430-9024 in Frisco, Texas.
Overview
Every tankless fault code is a safety report. Rinnai 11 means the board never proved ignition. Rinnai 14 means an overheat limit opened. Navien E109 means the exhaust or air pressure conditions fell outside the acceptable band. What none of those codes tell you is which of the four or five possible upstream conditions produced them, and that is where most failed repairs come from. Replacing an igniter because the display said ignition failure fixes nothing if the actual problem was manifold gas pressure collapsing under full fire.
Troubleshooting is therefore about sequencing measurements so each one eliminates a branch. Gas pressure first, because a supply problem imitates almost every combustion fault there is. We take a static inlet reading, then a reading with the unit at full fire, because natural gas systems that look fine at rest routinely sag once a 199,000 BTU appliance opens up. That single test resolves a meaningful share of the recurring ignition and flame-loss calls we see in Frisco, particularly in homes where a storage tank was converted to tankless without a gas load calculation.
Intermittent faults are the hard ones, and they need a different approach entirely. A unit that fails once a week and runs perfectly when the technician arrives cannot be diagnosed by observation. We read the control board's stored fault history, which on most brands retains the last several events with sequence information, and we correlate that against what the household was doing. A code that only appears on the first draw of a cold morning points somewhere very different than one that only appears when two showers overlap.
There is also a category of complaint that is not a fault at all. Cold water sandwich, the slug of cooled water sitting in the line from a previous draw, gets reported as a malfunction constantly. So does the normal minimum activation flow rate, which is why a trickling bathroom faucet will not fire the burner. Diagnosing those correctly matters, because the honest answer is that no part will fix them and the real remedy is a recirculation loop, a buffer tank, or nothing at all.
Diagnosis
Symptoms rarely point at a single cause. These are the ones we find most often on North Texas systems.
Cause: Rinnai 11, Navien E003 or Noritz 11 appear because the board never proved flame. A 1/2-inch branch on a long run cannot hold manifold pressure when a 199,000 BTU unit goes to full fire.
Fix: We manometer inlet and manifold pressure at full fire. If pressure falls outside the manufacturer's window, the fix is gas piping, not an igniter.
Cause: Rinnai 12 and equivalents usually come from a flame rod whose microamp signal has fallen below threshold due to oxide film, a degraded ground path, or reversed line polarity.
Fix: We clean the rod, measure the signal against the manufacturer's minimum, verify the ground, and check polarity at the receptacle before condemning any part.
Cause: Rinnai 14 and Navien E016 are scale codes most of the time. When they come back within weeks, the exchanger waterway is physically restricted or a thermistor is reading incorrectly.
Fix: We compare measured flow and temperature rise against rated output on a verified clean exchanger, then test thermistor resistance against its temperature curve.
Cause: Rinnai 10 and Navien E109 point at combustion air or exhaust. Causes include a blocked termination, an insect nest, condensate pooling in a sagging vent run, or a termination too close to an inside corner causing recirculation.
Fix: We inspect the full vent run for slope and support, clear obstructions, and measure termination clearances against the installation manual.
Cause: A partially blocked inlet filter screen or a fouled flow turbine drops sensed flow below the unit's minimum activation rate, so the burner simply never starts.
Fix: We clean the screen, verify the flow sensor output against actual measured gallons per minute, and confirm the fixture is drawing above the unit's minimum.
Cause: Cold water sandwich, minimum activation flow, and the two to eight second delay before hot water reaches a distant fixture all get reported as failures.
Fix: We demonstrate the behavior, explain the mechanism, and recommend a recirculation loop or buffer tank only where it genuinely solves the complaint.
Process
We ask for brand, model, the exact code, and the pattern: every time, only mornings, only when two fixtures run. Some codes have a safe homeowner reset and we will tell you when yours is one of them.
5-10 minutes
We enter the service menu and read the retained error log, which on most brands holds the last several events. Frequency and sequence often identify the cause before any tool comes out.
10 minutes
Static inlet pressure, then inlet and manifold pressure with the unit at full fire, compared against the manufacturer's stated operating window. Other gas appliances are cycled to check for interaction.
20 minutes
Burner flame observed, flame rod cleaned and its microamp signal measured, igniter gap checked, ground path and receptacle polarity verified.
20 minutes
Flow rate and temperature rise measured at a fixture, inlet filter screen inspected, flow sensor output compared to actual flow, and thermistor resistance checked against its temperature curve.
20 minutes
Full vent run checked for obstruction, slope, support and joint integrity, termination clearances measured, condensate trap and neutralizer examined.
15 minutes
You get the root cause in plain language, the measured values behind it, the repair options with firm pricing, and an honest note if replacement is the better call.
10 minutes
Benefits
Cost
| Factor | Effect on price | Range |
|---|---|---|
| Standard diagnostic fee | Covers the full measurement sequence. Applied toward the repair cost when you proceed with the work. | $89-$149 |
| Intermittent or non-reproducing fault | Chasing a fault that will not appear on demand takes longer and sometimes needs a return visit. | $50-$150 additional |
| Multiple units or a manifolded system | Cascade systems add controller communication and sequencing faults that single units do not have. | $60-$150 |
| Full combustion analysis | Analyzer readings of flue gas values, used when the fault points at air-fuel ratio rather than a discrete part. | $75-$150 |
| Gas load calculation | Required when pressure testing suggests the branch is undersized for the connected BTU load. | $75-$200 |
| Vent camera inspection | Used on concealed vent runs where obstruction is suspected but the run cannot be visually followed. | $100-$225 |
| After-hours diagnosis | Nights, weekends and holidays carry a dispatch premium, quoted before we roll. | Additional $75-$175 |
| Second-opinion review | Evaluating another contractor's diagnosis and quote, including verifying the measurements it was based on. | $99-$189 |
Typical total: $89-$189 diagnostic, credited toward the repair. Ranges, not quotes — a firm number needs eyes on the existing installation.
Comparison
| Code | What the unit is reporting | Most common actual cause |
|---|---|---|
| Rinnai 10 / Navien E109 | Air supply or exhaust blockage | Blocked termination, insect nest, or condensate pooling in a sagging vent |
| Rinnai 11 / Navien E003 / Noritz 11 | No ignition proven | Gas pressure sagging under full fire, or a fouled flame rod |
| Rinnai 12 / Noritz 12 | Flame lost after ignition | Weak flame rod microamp signal, poor ground, or reversed polarity |
| Rinnai 14 / Navien E016 / Noritz 14 | Overheat limit opened | Scale on the exchanger from 8-11 grain per gallon water |
| Rinnai 79 | Combustion or exhaust abnormality | Restricted intake, dirty burner, or air-fuel ratio drift |
| Rheem 11 | Ignition failure | Closed manual gas valve, low supply pressure, or igniter gap |
| Rheem 13 | Poor combustion detected | Fouled intake screen or partially blocked exhaust |
| Rheem 29 | Condensate drain blockage | Clogged trap or spent neutralizer media backing up |
| No code, water goes cold mid-shower | Nothing reported by the unit | Minimum activation flow, cold water sandwich, or a fouled flow sensor |
There are two ways to approach a faulting appliance. The first is recognition: you have seen this code before, last time it was the igniter, so it is probably the igniter. The second is elimination: you have a set of conditions that could each produce this code, and you design tests that remove them one at a time until one is left. Recognition is faster when it works and it is the source of nearly every repeat repair we get called in to sort out, because a code has several possible causes and pattern matching picks the one you happen to have seen most recently.
The practical consequence is that a good diagnosis produces a statement with a measurement attached to it, and a bad one produces a part number. If the conclusion of a visit is that the flow sensor was replaced, the useful question is what the flow sensor was measured to be doing that it should not have been. Sometimes the honest answer is that it was tested and failed, which is fine. Sometimes the honest answer is that it was the cheapest thing to try.
There is a related trap worth naming, because it accounts for a surprising share of misdiagnoses. Replacing a component and observing that the fault stops is weak evidence, particularly for an intermittent fault. Disturbing an assembly to fit a new part also reseats connectors, disturbs oxide films and moves wiring. A fault that occurred weekly and has not occurred in the four days since the repair has not been proven fixed, and treating it as proven is how the same unit comes back in a month.
Given several candidate causes, the useful test is not the one aimed at the most likely cause. It is the one that eliminates the largest number of candidates for the least effort. That is why gas pressure under full fire comes early on almost every combustion complaint: a single manometer reading either removes supply from consideration entirely, or identifies it as the cause and makes every downstream test unnecessary.
Read the stored fault history before touching anything
Free, non-invasive, and frequently decisive. A log showing five events clustered before seven in the morning in January describes a load-dependent supply problem before any instrument comes out of the bag.
Establish what is upstream of the appliance
Gas, water, air and electricity all arrive from somewhere else. Every one of them can produce faults that look like component failure, and all four are cheaper to check than any component is to replace.
Test under the conditions the fault occurs in
A unit at idle on a mild afternoon is not the system that failed at six on a January morning. Where the fault is load-dependent, the test has to reproduce the load: full fire, other gas appliances running, cold inlet.
Split the space rather than walking it
With eight candidates, a test that eliminates four is worth more than a test that confirms one. Water side versus combustion side is usually the first clean split, and measured flow against measured temperature rise makes it.
Verify the reference before trusting the reading
A measurement without a specification is a number. The rating plate and the service manual carry the acceptable windows, and those windows differ between brands and often between models within a brand.
Do not replace parts to test them
Substitution is a legitimate technique when a component genuinely cannot be tested in place. It is not a legitimate substitute for testing components that can be. Thermistors, flame rods, flow sensors and gas pressures are all directly measurable.
State the cause, then the remedy
If the cause cannot be stated in a sentence that includes a number, the diagnosis is not finished, and the correct thing to do is say so rather than quote a repair.
This sequencing is also why a proper diagnostic visit takes an hour or two rather than fifteen minutes. Most of that time is measurement rather than repair, and the value of it is that the repair which follows is the right one the first time.
A small number of readings do nearly all the work. Each one separates whole families of cause from one another, which is what makes them worth the time to take. Everything else is refinement.
| Measurement | Instrument | Typical reference | What it eliminates |
|---|---|---|---|
| Inlet gas pressure, static | Manometer | Around 7 in w.c. on residential natural gas | Meter and regulator problems |
| Inlet gas pressure, full fire | Manometer | Above the rating plate minimum, often 3.5-5 in w.c. | Branch sizing and load interaction |
| Manifold pressure | Manometer | Per the rating plate for the fuel | Gas valve and regulator drift |
| Flame signal | Microamp meter | Often 1-2 microamps minimum, healthy is several times that | Flame rod, ground path, line polarity |
| Thermistor resistance | Multimeter | Compared against the manufacturer's temperature curve | Sensor drift versus genuine overheat |
| Flow rate at a fixture | Measured fill against time | Above the minimum activation rate, commonly 0.4-0.6 GPM | No-fire complaints on low-flow draws |
| Temperature rise across the unit | Two thermometers or the service display | Compared with the GPM and BTU arithmetic | Splits water side from combustion side |
| Static water pressure | Gauge at a hose bib | 40-80 psi is the normal working band | Pressure-reducing valve failure |
The final row is included because it catches a fault that is invariably reported as a water heater problem. Pressure-reducing valves fail, and they fail toward low pressure as often as high. A house that has quietly dropped to 25 psi will not push enough flow through a tankless unit to reliably keep it above its minimum activation rate, and the complaint arrives as a heater that will not fire on the bathroom tap. The heater is faultless. The gauge on a hose bib is a two-minute check that finds it.
For intermittent faults the most valuable diagnostic instrument is a household's memory, and it is perishable. What changed and when is often sufficient to identify a cause outright, but only if somebody asks before four weeks of resets have blurred the sequence.
| What you noticed | What it commonly implicates |
|---|---|
| It started after a new gas appliance was fitted | Total connected load exceeding what the branch supports |
| It started after a power cut | Settings reset, vacation mode, or freeze damage during the outage |
| It started after a storm or high winds | Vent termination debris, or wind-driven recirculation at the terminal |
| It started in spring | Nesting in the exhaust or intake terminal |
| It started after work on the water main | Grit on the inlet filter screen or the flow turbine |
| It only happens on the first draw of the morning | Cold inlet driving the unit to full fire, exposing a supply limit |
| It only happens with two fixtures running | Capacity or gas supply, not a component |
| It has been getting slowly worse for a year | Scale, or a consumable such as neutralizer media |
| It began immediately after a repair | Something disturbed during that work |
Nothing on that table is a diagnosis. Every row is a hypothesis that then has to be tested with one of the measurements above. But starting from the right hypothesis is worth an hour, and the information costs nothing but the willingness to write things down when they happen rather than reconstructing them later.
If you are living with an intermittent fault now, keep a note by the unit: date, time, code shown, which fixtures were running, what the weather was doing, whether the furnace was on. Six entries in that format will usually identify the pattern on its own. Bring it to the phone call on (972) 430-9024 and the visit gets substantially shorter.
A genuine share of troubleshooting calls end with no repair, because the behaviour being reported is how the technology works rather than a defect. Saying so is part of the job. There is no component that fixes physics, and selling one anyway produces a customer who has paid to have the same experience.
| Complaint | Mechanism | What actually helps |
|---|---|---|
| Brief cold slug after restarting a tap | Cooled water sitting in the pipe from the previous draw | Recirculation or a buffer tank, or nothing |
| Bathroom tap will not fire the burner | Draw below the minimum activation rate | Open the tap further; check the aerator |
| Several seconds before hot water arrives | Pipe volume between the unit and the fixture | Recirculation on a timer, not continuous |
| Unit runs briefly with no tap open | Freeze protection or a scheduled recirculation cycle | Understanding the setting, not a repair |
| Ticking from pipework after a draw | Thermal expansion and contraction of the piping | Pipe support, not the appliance |
| Water dripping from the vent terminal in cold weather | Condensing exhaust meeting cold air | Nothing; it is expected on a condensing unit |
| Slight fan noise after the burner stops | Post-purge clearing the combustion chamber | Nothing; it is part of the cycle |
Where a complaint is genuine but the remedy is a design change rather than a repair — the wait for hot water at a fixture ninety feet from the appliance, for instance — the useful conversation is about what recirculation would actually achieve, at what cost, and with what effect on descaling interval. Running a recirculation loop continuously puts far more water through the exchanger than intermittent use does, and in 8 to 11 grain water that shortens the flush interval measurably. It is a real trade rather than a free improvement, and it should be described that way.
Want this priced for your specific Frisco home? Talk to a tankless specialist at (972) 430-9024.
Because the underlying condition was never corrected. A code is a safety trip, and resetting it clears the report without touching the cause. The classic Frisco example is an ignition code on a unit whose gas branch was never upsized after a tank-to-tankless conversion. The unit lights on a cold start, then loses pressure at full fire and locks out. Every reset buys a few days. Only measuring pressure under load and correcting the piping ends it.
Yes, and for a one-off code it is a reasonable first step. Turn the unit off at its power switch, wait about thirty seconds, and turn it back on, or use the reset function on the remote controller. What you should not do is reset repeatedly to keep a faulting unit running. Overheat and combustion codes exist because a safety limit was reached, and clearing them over and over lets whatever caused the trip keep doing damage. If a code returns more than twice, stop resetting and call.
Intermittent behavior almost always means something is marginal rather than broken. Common patterns: a flame rod whose signal sits just above threshold fails only when the burner runs a little rich; a gas branch that is barely adequate fails only when the furnace fires at the same time; a partially blocked filter screen fails only on low-flow draws. We read the stored fault log and correlate the failures with what the house was doing, which usually identifies which marginal condition is responsible.
Probably not. That is the cold water sandwich, and it is a plumbing characteristic rather than a fault. When a draw stops and restarts a minute later, the water sitting in the pipe between the unit and the fixture has cooled, so you get a brief cold slug before the newly heated water arrives. Nothing inside the heater causes it and no part fixes it. A recirculation loop with a dedicated return line, or a small buffer tank, eliminates it on long runs.
We work on Rinnai, Navien, Noritz, Rheem, Takagi and Bosch, which covers effectively all of the residential equipment installed across Frisco. Brand matters more than people expect, because the code taxonomies are entirely different and each manufacturer has documented model-specific behaviors. A Navien E016 and a Rinnai 14 both indicate overheat, but the service menu procedures, sensor resistance curves and acceptable gas pressure windows are not interchangeable.
Yes. If you proceed with the repair we recommend, the diagnostic charge comes off the total. We price diagnosis separately because a real diagnosis takes an hour or two of measurement, and pricing it into a repair creates pressure to find a repair whether or not one is warranted. Sometimes the correct outcome of a troubleshooting visit is that nothing needs replacing, and you should not have to pay for a part to get that answer.
Related
Repair
Error code on the display or no hot water at all — we diagnose the actual fault instead of guessing at parts.
Repair
Ignition lockout is the most misdiagnosed fault in tankless service. The igniter is rarely the part that failed.
Maintenance
Frisco water runs 8-11 grains per gallon. Calcium goes exactly where it does the most damage: the hottest surface in your heat exchanger.
Repair
The exchanger is the most expensive part in the unit and usually the most protected by warranty. We check coverage before we quote anything.
Maintenance
A documented condition assessment for home sales, warranty disputes, and units you inherited with the house and know nothing about.
Repair
Gas odor, water running over a finished floor, or no hot water in a freeze. Some things do not wait until Monday.
Background reading
Longer explanations of the tradeoffs behind this work, written for homeowners rather than trade readers.
Water Quality · 11 min read
Scale does not build evenly. It builds on the hottest metal in the system, which is the one surface a tankless water heater cannot afford to insulate. Here is the mechanism, the timeline and the fix.
Buying Guide · 10 min read
Tankless is not automatically the right answer. Here is the comparison with the inconvenient parts left in, including the three situations where a storage tank is still the better buy.
Troubleshooting · 11 min read
Your tankless unit did not get worse in December. The water got colder, and flow rate and temperature rise trade against each other. Here is the arithmetic, and the three real causes people mistake for it.
Coverage
We take tankless water heater troubleshooting calls throughout Collin County and Denton County. These are a few of the areas covered.
See every Frisco neighborhood and surrounding city we cover.
Call (972) 430-9024 and speak to someone who works on tankless systems every day. Open 24 hours a day, 7 days a week.
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Frisco, TX Tankless Pros