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

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

No ionization during the safety time, failed to ignite

Needs attention soon

Also written as failed to ignite, no flame, safety time, timeout

Quick answer

On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, EA is a fault. It means no ionization during the safety time, failed to ignite. A first thing to check: No gas supply.

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 gas valve. Bosch gives the required ranges as natural gas 5 to 14 inches of water column and liquid propane 11 to 14 inches of water column. These are the ranges for this control platform, and other Bosch platforms publish different numbers.

Checked 2026-09-26

What EA means on this model

The burner never showed a flame and the unit gave up after its ignition attempts. The cause is almost always on the gas supply side, such as no gas, air in the lines, low pressure or a damaged gas valve, with a weak ignition spark or failed flame sensor also possible.

Symptoms you will notice

The unit shows EA and will not produce hot water
The burner attempts to ignite several times and drops out
Clicking from the unit followed by no ignition

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:

No gas supply
Air in the gas lines
Low gas pressure
A damaged gas valve
A weak ignition spark
A failed flame sensor

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 gas valve. Bosch gives the required ranges as natural gas 5 to 14 inches of water column and liquid propane 11 to 14 inches of water column. These are the ranges for this control platform, and other Bosch platforms publish different numbers.
If the gas pressure is in range, check whether the combustion chamber is clean. An unserviced unit that has never been descaled is the usual reason the chamber is not clean.
Note that these ranges are measured at the unit, so a supply problem between the meter and the appliance will show up here.

Do not do these

×Do not keep cycling the unit trying to make it light. Three failed attempts and a code is the control protecting itself, and repeated forced attempts damage the igniter.
×Do not use these gas pressure ranges on a different Bosch control platform. The Greentherm platform publishes natural gas 3.5 to 10.5 and propane 8 to 13 inches of water column instead.
×Do not work on the gas valve without shutting the gas off and verifying it is off.

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 measurement at the appliance and any work on the gas valve belong to a licensed gas fitter.

What it costs to fix

Parts

Gas valve $180 to $400. Flame sensor $30 to $90. Igniter $60 to $150.

Labor

One to two hours to check pressure and the chamber, more if the valve or igniter has to come out.

DIY or technician