Takagi Tankless Problems: Codes & Fixes
The Short Answer: A Takagi (T-KJr2, T-K4, T-H3, and AO Smith-badged twins) that will not heat is usually an air/exhaust fault (10), ignition/flame failure (11/12), or a heat exchanger so scaled it overheats (14/16). Read the LED or controller, clear the intake and inlet screen, then stop if the code returns. Do not keep power-cycling a flooded or gas-starved burner.
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Search Intent & The Real Problem
Showers died mid-lather. The Takagi is either silent, clicking, or blinking a two-digit code. These units share DNA with older AO Smith tankless lines. The internet will tell you to “reset it” and “descale with vinegar.” Resetting a 11/12 without checking gas pressure or a 10 without checking the vent is how you glaze the igniter.
What other guides get wrong
Most roundups treat every Takagi like a Rinnai and skip the rating plate. Indoor T-H3 condensing units fail on condensate traps. Outdoor non-condensing T-KJr2 units fail on freeze, rain in the termination, and 1/2” flex gas connectors that cannot feed 140–190k BTU. “Vinegar in the cold inlet” without isolation valves sends acid into your house plumbing.
Step-by-Step Troubleshooting
Step 1: Read the code — do not guess
- What to look for: A two-digit code on the PCB LED or remote. Count flashes if there is no display.
- Required tools: Phone photo of the code and the model/serial on the rating plate.
| Code | Meaning | First check |
|---|---|---|
| 10 | Air supply / exhaust blockage | Termination, bird screen, condensate |
| 11 | Ignition failure | Gas cock open, igniter spark, lockout after 3 tries |
| 12 | Flame failure | Flame rod, low gas pressure, wind |
| 13 | Remaining flame | Gas valve not closing — shut gas, call a tech |
| 14 | Thermal fuse / overheat | Scale, blocked exchanger, dry fire |
| 16 | Outlet overheat | Scale, failed flow, mixing valve crossover |
| 31 / 32 | Outlet / inlet thermistor | Unplugged sensor, scale on probe |
| 99 | Other / board | Power quality, then licensed diagnosis |
A 13 is not DIY. Fuel is still flowing after the burner should be dead.
Step 2: Power, flow, and the inlet screen
- What to look for: Unit dark = GFCI or disconnect. Unit alive but no fire = flow below activation (often ~0.5–0.75 gpm on these). Grit in the inlet filter mimics “no hot water” with no code.
- Required tools: Flashlight, bucket + watch, inlet-filter wrench or channel locks with tape.
Close the cold isolation valve, pop the filter, rinse it. If you have no isolation valves, that is an incomplete install — hire the flush, do not unscrew the house shutoff and hope.
Step 3: Combustion air and venting (code 10)
- What to look for: Nested concentric vent, snow over an outdoor unit, a dryer vent jammed against the intake, PVC condensate trap full on T-H3. Indoor units need the manufacturer’s vent diameter and equivalent-length — a 3” run stuffed with 4” fittings will 10-lock forever.
- Required tools: Ladder, flashlight. No hose in the vent.
Step 4: Ignition and gas (codes 11/12)
- What to look for: Appliance shutoff parallel to pipe, sediment trap present, flex connector at least the size on the rating plate (many T-K4/T-H3 want 3/4” to the unit). Manifold pressure is a licensed manometer job: roughly 3.5–10.5” w.c. natural gas depending on model — confirm on the plate, never by ear.
- Required tools: Eyes on the connector and shutoff only.
If the unit was just installed and 11/12’s, the line is air-bound or undersized. That is a licensed purge and leak test, not a YouTube union-crack. See Don’t Purge a Water Heater Gas Line Yourself.
Step 5: Scale and overheat (14/16)
- What to look for: Hard water, skipped annual flush, longer heat-up, then a 16. Isolation-valve flush with the manufacturer’s pump kit is legitimate DIY. No service valves = licensed add-on plus descale.
- Required tools: Isolation valves, pump, Takagi-approved descaler.
A 16 that returns after a flush is a restricted exchanger or a failed thermistor. That is parts and combustion analysis, not a second jug of vinegar.
When DIY Ends: Calling a Licensed Pro
Stop at code 13, any gas-valve work, a wet PCB, a cracked heat exchanger, or a 10 that is actually a vent that was never legal. You may clean the inlet screen, reset once, and flush through factory isolation valves. Combustion, gas pressure, and board replacement are licensed. Takagi/AO Smith will not honor a heat exchanger that was run on a clogged condensate line or a 1/2” gas whip.
Frequently Asked Questions
Q: Why does my Takagi tankless have no hot water and no error code? A: Flow is below activation, the inlet filter is packed, or a shower cartridge is crossing over so net flow through the unit is near zero. Check flow at the unit, not at the head.
Q: How do I reset a Takagi tankless water heater? A: Power off at the switch or breaker for 10–30 seconds, then on. One reset after you fix the cause. Repeated resets on 11/12/16 destroy igniters and trip high-limits.
Q: Is a Takagi the same as an AO Smith tankless? A: Many residential AO Smith tankless units were Takagi platforms with different jackets and remotes. Use the model on the rating plate for parts, not the store brand.