Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code F7
Flame sensed while the unit is in standby
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, F7 is a fault. It means flame sensed while the unit is in standby. A first thing to check: A damaged gas valve.
- 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: Needs attention soon
- First safe check
- Check whether the flame sensor has been removed and cleaned or replaced. If it has not, the first step Bosch gives is to remove it and clean and inspect it.
What F7 means on this model
The heater thinks it is sensing burner flame while it is in standby, which is a gas safety problem rather than a nuisance code. Bosch points at a damaged gas valve or flame sensor, most likely from rain or condensate inside the unit.
Symptoms you will notice
- The unit shows F7 in standby
- The unit reports flame when no water is being drawn
- Evidence of water inside the unit
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:
- A damaged gas valve
- A damaged flame sensor
- Rain or condensate damage inside the heater
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 flame sensor has been removed and cleaned or replaced. If it has not, the first step Bosch gives is to remove it and clean and inspect it.
- If the sensor has already been cleaned or replaced, check whether the combustion chamber is wet.
- Look for where water entered the unit, because Bosch says that if the chamber is wet the heat exchanger needs to come out to check for leaks, and a leaking heat exchanger has to be replaced.
Do not do these
- ×Do not run the unit with F7 in place, because the control is reporting flame presence in a state where no flame should exist.
- ×Do not dry the unit out and return it to service without finding the water entry path, since the same water is what damages the gas valve.
- ×Do not clean the flame sensor with anything abrasive.
When to call a technician
Book a technician, do not keep resetting it
This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.
- 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. Bosch directs that the heat exchanger be removed to check for leaks, which is not a homeowner task.
What it costs to fix
Parts
Flame sensor $30 to $90. Heat exchanger $500 to $1,200 if it is found leaking. Gas valve $180 to $400 if water has reached it.
Labor
Two to four hours if the heat exchanger has to come out to look for leaks.
DIY or technician
A technician job. Bosch directs that the heat exchanger be removed to check for leaks, which is not a homeowner task.
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
- Models in scope
- Bosch Therm 250SX, GWH 635ES and 2400E gas tankless
- 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.