A2Bosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code A2

Overheat protection tripped, flue gas limiter or overheat fuse

Stop using it

Also written as overheat, flue gas limiter, overheat fuse

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, A2 is a fault. It means overheat protection tripped, flue gas limiter or overheat fuse. A first thing to check: Leak in the heat exchanger.

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: Press the reset button and see whether the code clears. A code that clears and comes back means the device cooled down and the underlying cause is still there.

Checked 2026-09-26

What A2 means on this model

A safety device in the upper half of the heater has opened the circuit because it got too hot. On early units it is a white overheat fuse wrapped around the heat exchanger that trips at about 355 degrees F, and on newer units it is a flue gas limiter in the upper corner of the cabinet that trips above 220 degrees F in the upper chamber. Before you act on it, check the platform: on the Greentherm T9000, A2 means an external temperature sensor is not detected, which is a wiring fault with no temperature threshold at all and no overheat involved.

Symptoms you will notice

The unit stops heating and shows A2
The reset button clears it for a short time and it comes back
Hot water goes cold and stays cold
Scorching, melting or a burnt smell around the heat exchanger or the upper cabinet
Banging or knocking from the unit when it tries to heat

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:

Leak in the heat exchanger
Bad overheat fuse or bad flue gas limiter
Bad wire connection to the protection device
Scale build-up on the heat exchanger holding heat against the device
On early units, the white fuse wire has crept too close to the copper heat exchanger over time and tripped on contact

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.

Press the reset button and see whether the code clears. A code that clears and comes back means the device cooled down and the underlying cause is still there.
Look around the copper heat exchanger for signs that exhaust gas has been leaking past it. Anything burned, melted or scorched means the unit should be taken out of service rather than reset.
On an early unit, confirm the white wire wrapped around the heat exchanger is still plugged into the black wires going to the control box.
On a newer unit, check that the flue gas limiter connections in the upper corner of the cabinet are tight.

Do not do these

×Do not jump out the fuse or sensor to get the burner back. Bosch is explicit that jumping it removes a critical safety circuit and is dangerous.
×Do not reset a unit that keeps returning to A2 without looking for exhaust gas leakage around the heat exchanger.
×Do not run the burner with the outer cabinet panels removed or with an overheat device disconnected.

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. The protection is a combustion safety device, and the difference between a tripped fuse and a leaking heat exchanger decides whether the unit is repairable.

What it costs to fix

Parts

Overheat fuse $30 to $90. Flue gas limiter $90 to $220. Heat exchanger $500 to $1,200 depending on model.

Labor

One to two hours to locate and test the device, more if the heat exchanger has to come out.

DIY or technician