HVACopediaRinnai · Tankless water heater troubleshooting
CO sensor voltage check (PCB connector C1)
What is going on
Voltage check on the CO sensor of a non-SENSEI Rinnai tankless, at the PC board. A meter reading tells you whether the sensor and its wiring look healthy.
Likely causes
- CO sensor failed or drifting out of range
- Bad connection at connector C1
Check in this order
- Measure at connector C1, between the red and black wires, pins 1 to 5, using DC volts.
- Normal reading is 1.9 to 2.1 VDC.
- During combustion, expect 2.5 to 3 VDC.
- Check the connector C1 for a loose pin or corrosion if readings are off.
- If readings are out of range, follow the CO sensor steps before replacing parts.
- Combustion and CO issues on a tankless are not a DIY repair, so use a licensed technician.
Applies to: Units with CO sensor
More tankless water heater guides
- Checking PC board fuses and thermistors
- CO sensor / combustion air fault (code number not printed on these pages)
- 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
Take it to the job
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Source: Troubleshooting Tankless Water Heater Diagnostic Codes, p. 37. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.