Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code C6
Fan rotation too low during operation
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, C6 is a fault. It means fan rotation too low during operation. A first thing to check: Debris, damage or water on the blower fan.
- 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: Book a technician
- First safe check
- Open the fan through the air scoop and inspect for debris, water or damage. The scoop is held by a metal bar across the middle of the unit with a screw at each end, plus two screws at the bottom of the scoop.
What C6 means on this model
The fan speed dropped below the acceptable range while the heater was running. Bosch publishes no single common cause for it, and the diagnostic is the same fan and flue check as C1.
Symptoms you will notice
- The unit shows C6 while running
- The unit shuts down partway through producing hot water
- Hot water goes cold mid draw on larger demands
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:
- Debris, damage or water on the blower fan
- A blocked flue pipe
- No single common cause is published for this code
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.
- Open the fan through the air scoop and inspect for debris, water or damage. The scoop is held by a metal bar across the middle of the unit with a screw at each end, plus two screws at the bottom of the scoop.
- Clear any debris and reset the code. If the code returns, the cause is not debris.
- Check the flue pipe for a blockage.
- Record whether the code only appears under heavy simultaneous hot water demand, which points at the induction system rather than the fan itself.
Do not do these
- ×Do not keep cycling the unit to get past a C6. If the fan is not turning at the speed the control expects, the gases are not being moved the way they need to be.
- ×Do not replace the fan without checking the flue, because a blocked flue produces the same code with a perfectly good fan.
When to call a technician
Worth scheduling, and worth your own checks first
You can usually keep living with this for a short while, but it will not fix itself. Do the safe checks below first, because a tripped switch or a blocked airflow is a five-minute fix that would otherwise cost you a service call.
- 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 anything past clearing visible debris.
What it costs to fix
Parts
Blower fan assembly $150 to $400.
Labor
One to two hours to inspect and clear.
DIY or technician
A technician job for anything past clearing visible debris.
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.