Rinnai Tankless Error Code 33: What a Heat Exchanger Outgoing Temperature Sensor Fault Means for East Cooper Homes
If your Rinnai tankless flashes Code 33, the board is saying it cannot trust the heat exchanger outgoing temperature sensor (the thermistor that reads water temperature as it leaves the heat exchanger). When that reading is missing, shorted, or out of range, the heater locks out rather than guess at how hot the water is leaving the HX.
Rinnai lists it as "Heat Exchanger Outgoing Temperature Sensor Fault." In plain English: check sensor wiring, measure resistance, clean scale off the sensor, or replace the sensor. That is licensed tech work — not a DIY ohm job.
This is related to, but not the same as, Code 32 (outgoing water temperature sensor fault). Code 32 is the sensor on the outgoing hot-water line / outlet side. Code 33 is the thermistor on the heat exchanger outgoing path. Same family of temperature-sensor faults. Different part. If you already read our Code 32 post, treat 33 as its HX-side cousin — don't swap the wrong sensor.
What usually trips Code 33
Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, the usual suspects look like this:
Loose, broken, or damaged wiring on the heat exchanger thermistor harness back to the PC board — vibration, a bumped connector, heat cycles, or someone working in the cabinet.
Scale build-up on the thermistor — coastal hard water and sediment coat the sensor tip so the reading drifts or fails. Common enough on East Cooper tankless that we look for it early.
A failed thermistor — open or shorted resistance that no longer matches what the board expects.
Less often, a board-side issue after other electrical problems on the circuit.
This is a temperature-sensor / thermistor fault. It is not a "flush and hope" scale code like LC by itself, and it is not ignition (11/12) or condensate (25). Don't treat it like a filter rinse.
Safe checks you can do
Keep it simple and stop before you open the cabinet, drain the unit, or try to ohm the sensor yourself:
Write down the code and when it started — after a storm, after recent service, only on hot-water demand, alongside another code, and so on.
Power-cycle once — turn the unit off at the controller, wait a minute, turn it back on. If 33 returns immediately, don't keep resetting.
Note recent work — someone flushed the system, moved the unit, stored bikes against a garage install, or had a power blip. That history helps.
If you smell gas, see water under the heater, or the unit is doing anything that feels unsafe — shut off gas and power and call. Don't keep cycling it.
Everything past that — draining the system, pulling the heat exchanger thermistor, cleaning scale, measuring resistance, reseating the O-ring, replacing the sensor — is licensed tech work. Rinnai's own procedure starts with killing water (and draining) before those steps.
When to call a pro
Call when:
Code 33 comes back after one reset
Hot water is weak, scalding, or gone
The code sits alongside other faults (including Code 32)
You're not set up to check the thermistor harness, resistance, or replace the HX outgoing sensor safely
A tech will typically check the wiring harness from the heat exchanger thermistor back to the PC board, inspect and clean scale on the sensor, measure resistance against Rinnai's chart, and replace the thermistor when the numbers say so. That's textbook guidance — not guesswork.
Local note for East Cooper homes
A lot of tankless units out here sit in garages, closets, or utility niches on Daniel Island and Mount Pleasant. Coastal hard water and humidity make scale on temperature sensors a regular visitor. If you've already dealt with Code 32 (outgoing water temperature sensor), you know the display is pointing at a temperature reading path. Code 33 points at the heat exchanger outgoing thermistor specifically. Fix that path; don't chase the whole heater — and don't confuse it with the outlet-side sensor.
Related reading if useful: Code 32 (averysplumbing.com), Code 52 (averysplumbing.com), Code 65 (averysplumbing.com).
Bottom line
Code 33 = heat exchanger outgoing temperature sensor fault. Safe home steps are limited: note the code, one reset, then a qualified tech for harness, scale, and sensor work. Don't force resets and don't open the water or gas side yourself.
I'm Christian with Avery & Sons Plumbing + Tankless. We work Rinnai tankless every week across Charleston, Daniel Island, Mount Pleasant, and East Cooper. If 33 is on your controller and you want it sorted, call (843) 305-5555 or visit averysplumbing.com.
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