What it means
The auxiliary thermostat A is stuck open. Coolant flow bypasses the thermal regulation, leading to slow warm-up and reduced engine efficiency.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Auxiliary thermostat A regulates coolant flow through its part of the engine’s cooling circuit. This code sets when the engine takes too long to reach its expected operating temperature because thermostat A remains open and allows coolant to circulate when it should be restricted.
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 takes longer than usual to warm up
- Heater blows cool or lukewarm air
How to diagnose it
- Review the freeze-frame data and compare live coolant-temperature readings from a cold start with the engine’s warm-up pattern.
- Check the coolant level when the engine is cold and look for leaks that could affect warm-up or temperature readings.
- Inspect the auxiliary thermostat A housing and nearby hoses for leaks, damage, or signs that the thermostat is not controlling flow.
- Compare temperatures in the relevant coolant branches as the engine warms to see whether coolant continues flowing through the A branch too early.
- If the temperature pattern points to thermostat A, test or remove it according to the vehicle’s service procedure and check its movement against the manufacturer’s specification.
Common fixes
- Correct a low coolant level and repair any leak found.
- Clear debris that is preventing thermostat A from moving, if serviceable.
- Replace auxiliary thermostat A if it remains open or fails testing.
- Replace a thermostat A housing or seal if damaged or leaking.
Can I keep driving?
A stuck-open thermostat A usually allows cautious driving, but the engine may run below its intended temperature, use more fuel, and provide weak cabin heat. Avoid prolonged trips in very cold conditions if visibility or comfort is affected by poor heater output.
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.