Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code EC
Loss of burner flame while burning
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, EC is a fault. It means loss of burner flame while burning. A first thing to check: Low gas pressure.
- 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
- Measure the gas pressure at the valve against the ranges for this platform, natural gas 5 to 14 and liquid propane 11 to 14 inches of water column.
What EC means on this model
The heater lost flame while running, and raises EC when the ionization signal disappears more than three times in less than thirty seconds. Causes sit in combustion quality: low gas pressure, air to fuel adjustment, dirty electrodes or a damaged gas valve. It is often accompanied by groaning during start-up.
Symptoms you will notice
- The unit shows EC and shuts down while heating
- Groaning noise during start-up
- Hot water runs out sooner than expected
- Burning smell
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:
- Low gas pressure
- Improper air to fuel, meaning CO2, adjustment
- Dirty electrodes
- A damaged gas valve
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.
- Measure the gas pressure at the valve against the ranges for this platform, natural gas 5 to 14 and liquid propane 11 to 14 inches of water column.
- Check the combustion chamber is clean, and descale with the heat exchanger removed if the unit has never been serviced.
- Listen during start-up for groaning, because Bosch associates that with this code and it distinguishes an EC from a plain flame failure.
Do not do these
- ×Do not adjust the gas and air mixture without a gas analyser, because CO2 adjustment is what sets the mixture and it is not a guess.
- ×Do not reuse the gas pressure ranges from another Bosch platform on this unit.
- ×Do not keep relighting a heater that drops the flame repeatedly, since each drop is an incomplete burn.
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. Gas pressure and mixture adjustment are licensed gas work.
What it costs to fix
Parts
Electrodes $25 to $80. Gas valve $180 to $400 if the valve is the cause.
Labor
One to three hours including a gas analyser check.
DIY or technician
A technician job. Gas pressure and mixture adjustment are licensed gas work.
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.