Bosch Therm 250SX, GWH 635ES and 2400E gas tankless error code CA
Water flow above the specified maximum
Quick answer
On Bosch Therm 250SX, GWH 635ES and 2400E gas tankless, CA is a fault. It means water flow above the specified maximum. A first thing to check: Unusually high incoming water 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: Book a technician
- First safe check
- Time the flow into a container of known volume to measure the actual rate. Over 10 GPM and the flow needs to be reduced.
What CA means on this model
The inlet flow sensor has measured more than about 10 gallons per minute through the unit. Bosch describes this as almost always a plumbing problem rather than a heater problem, usually unusually high incoming water pressure.
Symptoms you will notice
- The unit shows CA
- Hot water output is erratically cold when several fixtures run at once
- The unit struggles to deliver hot water during heavy demand
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:
- Unusually high incoming water pressure
- A pressure regulating or reducing valve that is failed or set too high
- A plumbing change that increased available flow
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.
- Time the flow into a container of known volume to measure the actual rate. Over 10 GPM and the flow needs to be reduced.
- Check the incoming water pressure and the setting on the pressure reducing valve if one is fitted.
- Note whether the fault only appears when several fixtures are opened at the same time, which points at the incoming supply rather than the heater.
Do not do these
- ×Do not attempt to override the flow limit in the control to run the unit at higher flow, since the rating exists for the combustion and heat transfer design.
- ×Do not chase this fault inside the heater first. Bosch states explicitly that it is usually an external plumbing issue.
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.
Measuring flow into a container is a homeowner task. Anything on the supply side beyond that is plumbing work.
What it costs to fix
Parts
Pressure reducing valve $30 to $120.
Labor
Fifteen minutes to measure flow, one to two hours to fit or reset a pressure reducing valve.
DIY or technician
Measuring flow into a container is a homeowner task. Anything on the supply side beyond that is plumbing 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.