CO sensor / combustion air fault (code number not printed on these pages)

What is going on

Page 37 gives steps for a CO sensor or combustion air problem. The code number is not printed on this page; it appears to continue after code 38. Also review codes 05 and 10 for the RH-180.

Likely causes

• Insufficient combustion air in the installation space
• Venting or clearance problem
• Wrong altitude DIP switch setting
• Debris in burner, air chamber or heat exchanger fins
• Faulty CO sensor or PC board

Check in this order

1. Verify 4 in. B-Vent materials were used and installed per the manufacturer.
2. Inspect the whole vent system inside and outside for clearance issues, sealed and secured joints.
3. Verify the space has enough combustion air for all gas water heaters in it. If the unit only runs with a door open, recalculate combustion air needs.
4. Verify DIP switches are set for altitude (see altitude table).
5. Remove the fan motor and inspect the fan wheel, housing and air duct for restriction. Reinstall and test.
6. Before replacing the CO sensor, ventilate the room, power the unit off and on, wait 30 minutes and try firing again.
7. With power, gas and water off, remove the burner manifold and inspect orifices. Remove the burner and clean debris from the burner and air chambers.
8. Inspect heat exchanger fins; if clogged, clean with compressed air at 120 psi or replace the heat exchanger.
9. Check CO sensor voltage at PCB connector C1 (see table). If still faulty, replace the CO sensor and PC board.
10. Review troubleshooting for codes 05 and 10 for the RH-180.

More tankless water heater guides

• Checking PC board fuses and thermistors
• CO sensor voltage check (PCB connector C1)
• Code 12 or 19 appears immediately after water flow starts (no flame in burner box)
• Cold water sandwich (temperature swings at fixture)
• Hot water still flowing with the unit off (bleed-down or cross-over)
• Water leaking from pump housing