FABosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code FA

Gas valve not closing the gas circuit

Stop using it

Also written as gas valve, gas leak, flame after shutdown

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, FA is a fault. It means gas valve not closing the gas circuit. A first thing to check: A damaged gas valve.

Applies to: Bosch Therm 250SX, GWH 635ES and 2400E gas tankless; Bosch Therm 250SX, GWH 635ES and 2400E gas tankless codes · Water Heater
Type of code: Fault; Urgency: Stop using it
First safe check: Watch the combustion chamber as the unit shuts down after water flow stops, and see whether there is flame when there should be none.

Checked 2026-09-26

What FA means on this model

The heater still thinks it is sensing flame after it has shut down, so the control is asking whether gas is still flowing. Bosch directs checking for moisture behind the cloud shaped plate held by three screws just above the gas valve, which is typically a sign of a leaking heat exchanger.

Symptoms you will notice

The unit shows FA after water flow stops
A flame visible in the combustion chamber when the unit is shutting down
Hot water that keeps running after the demand stops

Common causes

The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:

A damaged gas valve
A damaged flame sensor
Rain or condensate damage inside the heater
A leaking heat exchanger putting moisture behind the plate above the gas valve

What you can safely check

These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.

Watch the combustion chamber as the unit shuts down after water flow stops, and see whether there is flame when there should be none.
If there is no flame, remove the flame sensor to clean and inspect it and remove the heat exchanger to clean the burn chamber.
If there is flame, check the gas pressure against the ranges for this platform and check the gas valve is dry. Removing the cloud shaped plate held by three screws just above the gas valve shows whether there is moisture in the valve.
Moisture behind that plate is typically a sign of a leaking heat exchanger, and Bosch directs pulling the heat exchanger to check for leaks.

Do not do these

×Do not operate the unit again after an FA. A gas valve that does not close is not a fault that goes away on its own.
×Do not assume the flame sensor is the only problem, because the gas valve is the part that has to be trusted to shut gas off.
×Do not run the burner to test this with the unit open to the room.

When to call a technician

Stop using the appliance and call today

This one is a safety limit, not a nuisance. The control is shutting the unit down to prevent damage or a hazard, and it will keep doing that until the underlying cause is fixed. Running it anyway risks the expensive failure it was designed to prevent.

This is a fuel-burning appliance, so treat anything involving a burner, gas valve or flue as technician work. If you smell gas at any point, leave the property and call the gas utility from outside rather than continuing to diagnose.
If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.

A technician job, and a gas safety one. Bosch technical support should be contacted rather than the unit being put back in service.

What it costs to fix

Parts

Gas valve $180 to $400. Flame sensor $30 to $90. Heat exchanger $500 to $1,200 if it is leaking.

Labor

Two to four hours, more if the heat exchanger has to be pulled.

DIY or technician