E9Bosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code E9

Overheat sensor opened circuit

Stop using it

Also written as overheat sensor, ECO, scale, banging

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, E9 is a fault. It means overheat sensor opened circuit. A first thing to check: Scale, meaning mineral deposit, build-up on 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: Check whether the code resets once the unit has cooled down. If it does not reset when cold, the sensor itself is faulty and needs replacing.

Checked 2026-09-26

What E9 means on this model

The green and white overheat sensor on the upper right corner of the heat exchanger has opened because it sensed over 220 degrees F. It resets itself once the unit cools but the code still needs the reset button. Bosch says the usual cause is scale build-up in the heat exchanger, often with banging or knocking from the unit.

Symptoms you will notice

The unit shows E9 and will not heat
Banging or knocking noise from the heater
The code clears after the unit cools but returns
The code will not clear even after the unit is cold

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:

Scale, meaning mineral deposit, build-up on the heat exchanger
A faulty overheat sensor
A heat exchanger that is not shedding heat because of deposits

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.

Check whether the code resets once the unit has cooled down. If it does not reset when cold, the sensor itself is faulty and needs replacing.
Ask whether the appliance has ever been maintained. If it has not, the answer is almost certainly scale, and the fix is a descale and a heat exchanger removal to clean the combustion chamber.
When cleaning, hose out the heat exchanger and brush debris off the burner bed, and watch the ceramat layer, which is the felt-like coating on the burner. If the ceramat is damaged, Bosch says the burner should be replaced.

Do not do these

×Do not jump out the overheat sensor. Bosch states that this removes a critical safety circuit and puts the heater at risk of fire or explosion.
×Do not reset and keep using the unit on a recurring E9, because each trip means the heat exchanger is not shedding heat.
×Do not brush the burner hard enough to cut the ceramat, because a damaged ceramat means the burner has to be replaced.

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. This is a fire and explosion protection and Bosch is unambiguous about not bypassing it.

What it costs to fix

Parts

Overheat sensor $40 to $120. Descaling service $150 to $350. Burner $120 to $300 if the ceramat is damaged.

Labor

Two to four hours to descale with the heat exchanger out, plus the replacement part if the sensor has failed.

DIY or technician