Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code A7
Hot water temperature sensor fault or out of range
Which models this applies to. This page covers the Bosch Therm 250SX, GWH 635ES and 2400E gas tankless codes.
What is specific to the Therm 250SX, GWH 635ES and 2400E gas tankless platform
Condensing gas tankless platform with a condensing heat exchanger, an induction fan and a flue gas limiter. Four of its codes, A2, A7, A9 and C1, mean something completely different on the Greentherm T9000 platform, and the gas pressure ranges published for it differ from the T9000 ranges, so the code has to be read against the model family rather than as a Bosch code.
Your control board prints the code as text, for example F01 or EE3, so you read it off the display rather than counting blinks. The printed number is the fault itself, not a position in a table.
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, A7 is a fault. It means hot water temperature sensor fault or out of range. A first thing to check: The sensor is reading temperatures outside its normal range because the appliance genuinely needs service.
- 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 or maintained. If it has never been cleaned, the first move is a descale and a heat exchanger removal to clean the combustion chamber, not a sensor swap.
What A7 means on this model
The sensor clipped to the hot water pipe at the bottom of the unit is reporting a temperature the control does not believe. Bosch warns against assuming the sensor is at fault, because on a scaled heat exchanger the sensor is correctly reporting a real overheat. On the Greentherm T9000, A7 means the heat exchanger outlet water sensor is not detected, which is a wiring fault rather than a temperature reading.
Symptoms you will notice
- The unit shows A7
- Hot water is too hot, too cold or erratic
- The unit will not hold temperature
- The unit shuts down while running
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 sensor is reading temperatures outside its normal range because the appliance genuinely needs service
- Corroded sensor contacts or a loose connection
- A failed sensor
- Scale build-up in the heat exchanger, which the control is correctly reporting as a temperature the water should not reach
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 or maintained. If it has never been cleaned, the first move is a descale and a heat exchanger removal to clean the combustion chamber, not a sensor swap.
- Confirm the two red wires are tight at the sensor and that the sensor is still firmly clipped to the top, middle of the horizontal run of hot water pipe.
- Look at the sensor and its contacts for corrosion, and clean the contacts with light sandpaper or an eraser.
- Ohm out the sensor against the temperature sensor resistance chart in the manual and compare it to the actual water temperature. A sensor that does not track the chart is the failed part.
Do not do these
- ×Do not assume a bad sensor the moment the code appears. Bosch warns the error may be legitimate and the heater may be correctly reporting a temperature that is out of range.
- ×Do not swap the sensor without checking the resistance against the chart, because a healthy heater with a marginal sensor and a scaled heat exchanger produce the same code.
- ×Do not leave the sensor unclipped on the pipe, because an unclipped sensor reads the cabinet air instead of the water.
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 resistance check is straightforward with a meter, but reaching the sensor means draining the unit, so this is normally a technician job.
What it costs to fix
Parts
Hot water temperature sensor $25 to $80.
Labor
Thirty to sixty minutes to test and replace once the appliance is drained.
DIY or technician
The resistance check is straightforward with a meter, but reaching the sensor means draining the unit, so this is normally a technician job.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Bosch Therm 250SX, GWH 635ES and 2400E gas tankless codes
- How this model reports the code
- Printed on the control display
- Models in scope
- Bosch Therm 250SX, GWH 635ES and 2400E gas tankless
- How this platform differs
- Condensing gas tankless platform with a condensing heat exchanger, an induction fan and a flue gas limiter. Four of its codes, A2, A7, A9 and C1, mean something completely different on the Greentherm T9000 platform, and the gas pressure ranges published for it differ from the T9000 ranges, so the code has to be read against the model family rather than as a Bosch code.
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.