8EBosch · Therm 250SX, GWH 635ES and 2400E gas tankless · Water Heater

Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code 8E

Electrical noise reported on the control board

Worth keeping an eye on

Also written as noise, board, 8 E

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, 8E is a fault. It means electrical noise reported on the control board. A first thing to check: A wrong combination of buttons being pressed on the unit.

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: Worth keeping an eye on
First safe check: Reset the unit and see whether the code goes away. If it does, verify the power supply and run the unit through a full heat demand to see whether it returns.

Checked 2026-09-26

What 8E means on this model

The control board has picked up electronic noise. As with E0, a single occurrence does not mean a bad board, and it can be triggered by the wrong combination of buttons being pressed.

Symptoms you will notice

The unit shows 8E
The unit works after a reset
The code appears repeatedly
The code appears right after someone was changing a setting

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 wrong combination of buttons being pressed on the unit
A defect on the circuit board
A supply or grounding condition that lets noise onto the board

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 code goes away. If it does, verify the power supply and run the unit through a full heat demand to see whether it returns.
Ask what was being done to the unit immediately before the code appeared, since a button combination is a documented cause.
Check the supply at the control box against the 120 volt AC plus or minus 10 percent that Bosch specifies for this platform.

Do not do these

×Do not replace the board on a single 8E. Bosch states a single occurrence does not indicate a bad board.
×Do not keep resetting a code that keeps returning without having the supply checked, because supply noise and a board fault look the same from the panel.

When to call a technician

No call needed yet, but write it down

Note the date and the exact code, and watch whether it comes back. If it turns into a repeat fault or starts affecting performance, that is when it becomes worth a technician.

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.

Resetting and monitoring is a homeowner task. A board replacement is a technician job.

What it costs to fix

Parts

Control board $200 to $450 if replacement is needed.

Labor

Thirty minutes to reset and monitor, one to two hours if the board has to be replaced.

DIY or technician