What it means
The auxiliary thermostat A is stuck closed. Coolant cannot flow through the affected branch, risking overheating in the bypassed path.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Auxiliary thermostat A controls coolant flow through its branch of the cooling system. This code sets when the expected temperature or flow response indicates that thermostat A is not opening, restricting circulation through the affected branch and risking overheating there.
Common causes
Most likely first.
- Thermostat element failed
- Wax pellet aged - element non-responsive
- Carbon or debris buildup preventing free movement
Symptoms
- Check-engine light
- Engine temperature rises or overheating
- Heater may blow cool air
How to diagnose it
- Review freeze-frame data and watch live coolant temperature from a cold start for a rapid rise or an overheating trend.
- Check the coolant level only when the engine is cold, and inspect for leaks that could cause abnormal temperatures.
- Inspect thermostat A’s housing and the hoses for its branch for kinks, blockage, leaks, or other visible damage.
- Compare temperatures across the hoses entering and leaving the A branch as the engine warms to check for restricted flow.
- If evidence points to thermostat A, test its movement and opening response using the vehicle’s service procedure and manufacturer’s specification.
Common fixes
- Repair a leak and refill the cooling system as specified for the vehicle.
- Clear a blockage or correct a kinked hose in the auxiliary thermostat A branch.
- Replace auxiliary thermostat A if it fails to open during testing.
- Replace a damaged thermostat housing or seal.
Can I keep driving?
Do not continue driving if the temperature rises above normal or an overheating warning appears; stop safely and let the engine cool. Restricted flow through thermostat A can cause local overheating and serious engine damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.