What it means
The second coolant bypass valve is stuck in the open position. Coolant always bypasses the second radiator path.
At a glance
- Usually lights the check-engine light
How it works
Coolant bypass valve B directs coolant through or around a cooling-system path as the engine warms and runs. The engine control module detects that valve B remains open when it expects the valve to close, so coolant continues through the bypass path.
Common causes
Most likely first.
- Valve seized open by sediment
- Broken return spring
Symptoms
- Check-engine light
- Engine temperature may fluctuate
- Heating or warm-up may be affected
How to diagnose it
- Record freeze-frame data and compare coolant-temperature readings during warm-up to confirm when the fault occurs.
- With the engine cool, inspect bypass valve B and its connectors for leaks, damage, contamination, or a displaced linkage.
- Use a scan tool to command valve B through its available positions and see whether coolant temperature responds as expected.
- Check whether sediment is restricting valve B or its mechanism and whether the valve moves freely.
- Inspect the return spring for breakage or loss of tension, then test the valve actuator and control circuit to the manufacturer's specification.
Common fixes
- Clean debris from valve B if it is preventing movement.
- Repair a damaged connector or wiring to valve B.
- Replace the broken return spring if serviceable separately.
- Replace bypass valve B if it is seized open.
Can I keep driving?
Short, gentle trips may be possible if the temperature gauge stays normal, but monitor it closely and avoid heavy loads or long drives. Stop if the engine overheats; continued incorrect coolant routing can worsen temperature fluctuations and affect warm-up or cabin heat.
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.