Rinnai RH180 Hybrid Code 38
Carbon monoxide or flammable vapour sensor fault. On the RH180, code 38 is the CO or FV sensor — the carbon monoxide sensor. This is the single most important difference between this appliance and the rest of the Rinnai range, where the same number is an exhaust gas temperature sensor. If you have arrived here from a page or a forum post about Rinnai code 38, that page is describing a different component on a different product. Rinnai's remedy is three lines — check the sensor wiring for damage, measure the sensor's resistance, replace the sensor.
Call a proThe verdict
Call a licensed plumber
The part is not expensive and the fix is short, but the component is the carbon monoxide sensor on a gas appliance, and what is broken is the safety device rather than the heating. Have it seen quickly, and do not keep the unit running in the meantime on the grounds that hot water still works.
Typical cost: $150–$700
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the RH180 Hybrid — not averaged across the brand.
Source: Rinnai RH180 Operation and Installation Manual (100000319) (p. 36), read Sep 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Turn the unit off rather than running it with the CO sensor unreliable.
- Check you have a working carbon monoxide alarm in the house, and replace the battery if you are not sure.
- Call a licensed professional and say clearly that it is code 38 on an RH180 — the CO sensor. On any other Rinnai that number is an exhaust temperature sensor, and the wrong part gets ordered otherwise.
Frequently asked questions
Every other site says Rinnai code 38 is an exhaust gas temperature sensor. Who is right?
Both, for different appliances. On Rinnai's tankless models — SENSEI, SENSEI RX and the Non-SENSEI range — code 38 is the exhaust gas temperature sensor. On the RH180 hybrid, Rinnai's own manual heads the same number "CO or FV Sensor". This is the clearest example on the whole site of why a code has to be read against the exact model.
Can I keep using the water heater until the technician comes?
We would not. The appliance still heats water, but the sensor that watches for carbon monoxide is the part reporting a fault. Turn it off, and make sure the house has a working CO alarm independent of the heater.
Code 38 on other Rinnai series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
5 other Rinnai RH180 Hybrid codes point at a sensor too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: