Rinnai code 38: exhaust gas temperature sensor fault

The exhaust gas temperature sensor watches how hot the exhaust leaving the heater is. That protects the vent and helps the heater catch combustion problems. Code 38 means the sensor, or what it is reading, is outside the expected range.

The meaning and steps below come from Rinnai's own diagnostic code guide for its tankless water heaters.

Sensor checks

1. Inspect the wiring harness at the sensor and at the control board.
2. Remove the sensor and measure its resistance. Rinnai lists values such as about 11,400 to 14,000 ohms at 59°F; the reading should fall as the sensor warms.
3. If the readings are right, reinstall it in the exhaust port and run the heater.

If code 38 continues

• Confirm the heater runs on the gas type shown on its rating plate, and that any gas conversion was done exactly per Rinnai's instructions.
• Check clearances around the heater and vent terminals.
• Confirm the parameter or DIP switch settings for the installation.
• Have manifold gas pressures checked with a manometer.
• Inspect the burner and heat exchanger fins for debris, and the venting for correct length and clearances.
• Replace the sensor if nothing else explains the code.

RH180 hybrid models

On Rinnai's RH180 hybrid tank-tankless heater, the diagnostic guide ties code 38 to the carbon monoxide sensor, with combustion air, B-vent venting and gas pressure checks. Make sure working CO alarms are installed in the home.

Before you call

Have the model and serial number from the rating label ready, and note whether your unit is a SENSEI or an older non-SENSEI model. Tell the technician the exact code, when it appears and any recent service or parts replacement. That often saves a second visit.

See every code on our Rinnai error codes page.