E2Bosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code E2

Inlet temperature sensor fault

Book a technician

Also written as inlet sensor, cold water sensor

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, E2 is a fault. It means inlet temperature sensor fault. A first thing to check: The inlet sensor reading temperatures outside normal parameters.

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: Ask whether the unit has been serviced. If it has never been cleaned, descale the heat exchanger and clean the combustion chamber first.

Checked 2026-09-26

What E2 means on this model

The cold water temperature sensor feeding the heat exchanger is reporting something the control does not accept. Bosch does not want a technician assuming a bad sensor, because the reading may be correct.

Symptoms you will notice

The unit shows E2
Hot water delivery is erratic
The unit will not reach set temperature

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:

The inlet sensor reading temperatures outside normal parameters
A failed inlet temperature sensor
Corroded contacts or a loose connection
Scale build-up in the heat exchanger

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.

Ask whether the unit has been serviced. If it has never been cleaned, descale the heat exchanger and clean the combustion chamber first.
Confirm the two red wires are tight at the sensor and the sensor is firmly clipped to the pipe.
Use the sensor swap test Bosch describes. The hot and cold sensors are identical, so you can swap the sensors and not the wires. If the error changes to A7, the sensor you moved to the cold side is the failed one.
Compare the measured resistance against the temperature chart in the manual. Bosch also suggests clipping both sensors to the same pipe and comparing them, since two healthy sensors should read roughly the same resistance.

Do not do these

×Do not assume a bad sensor the moment E2 appears, because the error may be a real reading.
×Do not swap the wires when doing the sensor swap test. The test is sensors, not wires.
×Do not leave either sensor clipped to the wrong pipe after the test, because the heater will then run on mismatched inlet and outlet readings.

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.

The sensor swap test is a good diagnostic, but it needs the unit drained and the pipes disconnected, so it is normally a technician job.

What it costs to fix

Parts

Inlet temperature sensor $25 to $80.

Labor

Thirty to sixty minutes to test and replace.

DIY or technician