Heater goes out after 20min error code 12 appears

Please always include full 'make model (what it is)'.

If it is a Rinnai

Rinnai Gas Heater fault code 11 indicates a no ignition or no gas supply on your gas heater system. ... A gas supply issue may be present if the flame on the gas heater pilot drops in pressure, of if your gas heater does not work then there is no gas supply present.Feb. 10, 2020

What is Rinnai Error Code 11 or 12? - Gas Heater Service ...

There is a video as well

https://www.google.com/search?q=Gas+heater+giving+error+code+11

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get a gas fitter in to fix the problem as gas is less forgiving than electricity

You don't mention any error codes displayed. If this is the case I would start with a new main PCB. It should try and light for approx. 10 seconds and then go to lockout with an error code 11 if it was a gas supply issue. This is all controlled by the PCB hence my opinion on a PCB fault.

Mysolution relates only if you are on LPG/bottleg gas. I have had many problems with the gas regultor actually on the initial supply line near the bottles sticking. Error code 11 is usually simply a gas supply issue and not a fault with the heater itself. Just rule out that before you start messing with the heater itself. Causes inside the heater would be either gas valve (unlikely) or PCB. Both are not normally intermittent. Try running something else on gas if you have anything and then turn on the heater.

Code 11 is that the unit is trying to light but does not sense a flame after 5 seconds. Usually means you have run out of gas if you are bottled gas. Check your bottles are turned on if you are on LPG, you might need to change over manually. Have you anything else on gas?

Three main causes.

Short circuit on the low voltage side of the wiring (12V) This will show as an immeadiate error code 12 befoe the thing even tries to light.

Faulty earth on the low voltage side shows same as above.

A faulty gas regulator or partial blockage of gas supply that allows the unit to fire, but will not maintain sufficient gas supply to keep it alight. This will happen after it has lit, maybe erratically with the occasional error code 11 if you try and re-light immeadiately.

2000 is the year of manufacture

11 means no gas / failed to ignite - check the bayonet is fully plugged in properly, they can be hard to plug/unlpug - otherwise you wilol have to take it in for service.

No conversion parts available since 2005.

Error code 12 is a genaric "Flame Failure" code. I suspect that the flame is not hitting the flame sensor or that the sensor is bad. You will need to get to the sensor located on the far left hand side of the unit. If you have the owners manual, it's part #123. Remove it and clean it off. Make sure it is placed back in the heater properly aligned and all the way in. Then try and light it. If it does not stay lit, it may need to be replaced but check out the burner tray for debris not allowing the flame to travel all the way to the sensor. You will have to remove many parts to get to the burner assembly.