Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code C1
Fan rotation too low at start-up
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, C1 is a fault. It means fan rotation too low at start-up. 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 air scoop on the front of the fan to inspect it. The fan sits in the bottom half of the unit behind a black scoop, held by a metal bar across the middle of the unit, which is held by two screws.
What C1 means on this model
The fan speed was below the range the control accepts when the unit tried to start. A single occurrence right after another code was reset, or after the heater was cycled quickly while the fan was still purging, is expected and not a concern. On the Greentherm T9000, C1 means a vent blockage was detected, which has nothing to do with fan speed.
Symptoms you will notice
- The unit shows C1 at start-up
- The unit attempts to start and stops
- Hot water demand is met inconsistently
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
- A previous error that was not cleared properly
- The heater being cycled on and off quickly while the fan is still running
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 air scoop on the front of the fan to inspect it. The fan sits in the bottom half of the unit behind a black scoop, held by a metal bar across the middle of the unit, which is held by two screws.
- Look for debris, water or physical damage to the fan. Clear debris and reset the code; if the code comes back, keep going.
- Check the flue pipe for a blockage.
- Note whether the code appeared only once, right after another code was reset or after rapid cycling. A single occurrence in that situation is expected behaviour.
Do not do these
- ×Do not chase a single C1 that arrived right after another reset or rapid cycling. Bosch treats that as not a concern.
- ×Do not run the unit repeatedly with a debris-packed fan, because the fan is what moves the combustion gases out and the sequence is checking that it actually did.
- ×Do not assume a damaged fan is the only damaged part after water ingress, since other components are usually affected too.
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.
Opening the fan and clearing debris is achievable, but the flue inspection and any fan replacement is a technician job.
What it costs to fix
Parts
Blower fan assembly $150 to $400. Fan motor $180 to $350.
Labor
One to two hours to open, inspect and clear the fan and check the flue.
DIY or technician
Opening the fan and clearing debris is achievable, but the flue inspection and any fan replacement is a technician job.
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
- How this model reports the code
- Printed on the control display
- Models in scope
- Bosch Therm 250SX, GWH 635ES and 2400E gas tankless
- How this platform differs
- 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.
- 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.