Code 12 or 19 appears immediately after water flow starts (no flame in burner box)

What is going on

When code 12 or 19 shows up the instant water starts to flow, and you never see a flame in the burner box, I think short circuit before I think burner trouble. The unit is not even getting as far as lighting, so something on the low voltage DC side is pulling the circuit down. A harness chafed against the cabinet, a bad water flow control or a stuck bypass valve are what I find most often. This applies to both SENSEI and Non-SENSEI series. Power off the heater before you unplug anything.

Likely causes

• Electrical short in a wiring harness
• Short in the water flow control or bypass valve
• Short in any other component on the DC circuit
• Pinched or chafed wire against the cabinet or burner box

Check in this order

1. Turn off power to the water heater before unplugging or reconnecting the fan motor. Failure to do so can short out the fan motor
2. Inspect harnesses for chafing, burn marks and pinched wire
3. Unplug one component at a time and try to cycle the water heater on
4. If the water heater cycles three times after a component is unplugged, the short is in that device
5. Replace the faulty device or repair the harness
6. Reconnect everything and run a full draw to confirm the code is gone
7. Call a licensed technician if the unit still faults

More tankless water heater guides

• Checking PC board fuses and thermistors
• CO sensor / combustion air fault (code number not printed on these pages)
• CO sensor voltage check (PCB connector C1)
• 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