A9Bosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code A9

Outlet temperature sensor not seeing the set temperature

Book a technician

Also written as outlet temperature, set temperature

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, A9 is a fault. It means outlet temperature sensor not seeing the set temperature. A first thing to check: Temperature undershoot or overshoot at the outlet.

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: Book a technician
First safe check: Establish whether the appliance has ever been serviced. If it has not, descale the heat exchanger and clean the combustion chamber before touching the sensor.

Checked 2026-09-26

What A9 means on this model

The outlet sensor is not reporting the temperature the control expected, from a temperature undershoot or overshoot. It can flash while the heater keeps running or shut the heater down, and the control board software version affects how it presents. On the Greentherm T9000, A9 means output power is too low, which is a gas pressure fault and has nothing to do with the outlet sensor.

Symptoms you will notice

The unit shows A9, sometimes while still producing hot water
Outlet temperature is under or over the setpoint
The unit shuts off and needs a reset
Hot water temperature swings from one draw to the next

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:

Temperature undershoot or overshoot at the outlet
A bad or mispositioned hot water temperature sensor
Scale build-up in the heat exchanger
A difference in control board software version

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.

Establish whether the appliance has ever been serviced. If it has not, descale the heat exchanger and clean the combustion chamber before touching the sensor.
Verify the two red wires are tight and the sensor is clipped to the top, middle of the horizontal section of the hot water pipe.
Check the sensor and its contacts for corrosion and clean them with light sandpaper or an eraser.
Compare the measured resistance of the sensor against the temperature chart in the manual for the actual water temperature. A reading off the chart means the sensor is the failed part.

Do not do these

×Do not reset and ignore a recurring A9. The heater may keep running with the code flashing, which hides a real temperature problem.
×Do not reposition the sensor off the top of the pipe in an attempt to change the reading.
×Do not assume the board is at fault before ruling out the sensor and a scaled heat exchanger.

When to call a technician

Worth scheduling, and worth your own checks first

You can usually keep living with this for a short while, but it will not fix itself. Do the safe checks below first, because a tripped switch or a blocked airflow is a five-minute fix that would otherwise cost you a service call.

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 once draining is involved, which it usually is for this sensor location.

What it costs to fix

Parts

Hot water temperature sensor $25 to $80.

Labor

Thirty to sixty minutes to test and replace.

DIY or technician