When an RV water heater pilot lights but won't stay lit after releasing the control knob, the cause is almost always a failed thermocouple — the safety sensor that detects pilot flame heat and tells the gas valve to stay open. Less common causes include a dirty pilot orifice, low propane pressure, or a failing gas valve.
Most likely causes (in order of likelihood)
- Failed or weak thermocouple — the #1 cause, accounting for the large majority of pilot-out service calls.
- Dirty or partially clogged pilot orifice — flame is too small or yellow to fully heat the thermocouple.
- Thermocouple not positioned correctly in the pilot flame.
- Low propane pressure — regulator out of spec, low tank, or two-stage regulator switching incorrectly.
- Draft — wind blowing across the burner area on units with damaged covers.
- Failed gas valve (final cause to suspect — relatively rare).
Diagnostic steps (in order, free/cheap before expensive)
- Hold the control knob in for the full 30-60 seconds the manual specifies. Many 'won't stay lit' calls are users releasing the knob too soon.
- Inspect the pilot flame. It should be steady, blue, and wrapping around the upper third of the thermocouple. A yellow, weak, or wavering flame means dirty orifice or low pressure.
- Clean the pilot orifice. Shut off propane, remove the pilot assembly, and clear the tiny orifice hole with a thin wire or compressed air. Do not use a drill bit — you can enlarge it and create a dangerous flame.
- Verify thermocouple position. The tip must sit directly in the pilot flame, not above or beside it.
- Test the thermocouple. With it in the flame for 30+ seconds, the millivolt output should be 18-30 mV. Below 12 mV, replace it. Replacement parts run $10-$25.
- If thermocouple and orifice are good and pilot won't stay lit, suspect the gas valve — tech-level repair.
DIY vs. call a tech
All steps are DIY for anyone willing to shut off propane, work carefully, and follow the service manual. Thermocouple replacement takes 20 minutes once you've done it once.