Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code E0
Control board software or hardware failure
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, E0 is a fault. It means control board software or hardware failure. A first thing to check: A bad circuit board.
- 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
- Reset the unit and see whether the behaviour returns to normal. A single E0 that clears is worth monitoring rather than replacing a board over.
What E0 means on this model
The control board has reported an internal failure. A single E0 does not mean a bad board, but repeated E0 codes do. It can also come from a button combination entered by mistake while resetting or changing a setting.
Symptoms you will notice
- The unit shows E0
- The unit may or may not still heat
- The code appears after someone was pressing buttons on the unit
- Repeated E0 codes
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 bad circuit board
- A wrong combination of buttons pressed while resetting an error or changing a setting
- A supply voltage problem at the unit
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.
- Reset the unit and see whether the behaviour returns to normal. A single E0 that clears is worth monitoring rather than replacing a board over.
- Check the unit is plugged into an outlet on its own dedicated circuit, and if it is not, move it to one that is.
- Measure the supply at the control box. Bosch specifies 120 volts AC with a plus or minus 10 percent tolerance, so anything outside roughly 108 to 132 volts is a power supply problem rather than a board problem.
Do not do these
- ×Do not replace the control board on a single E0. Bosch states a single occurrence does not indicate a bad board.
- ×Do not leave the unit on a shared circuit, because supply variation is one of the documented causes of this code.
- ×Do not keep resetting a control that will not hold a reset, because that points at the board or the supply, not at the code.
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 for the voltage check and the board. The dedicated circuit check is something the homeowner can confirm.
What it costs to fix
Parts
Control board $200 to $450.
Labor
One to two hours to diagnose and replace.
DIY or technician
A technician job for the voltage check and the board. The dedicated circuit check is something the homeowner can confirm.
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.