What it means
Coolant level sensor signal voltage above the upper limit — short to reference voltage or open signal return.
At a glance
- Usually lights the check-engine light
How it works
The engine coolant level sensor or switch sends the expansion-tank level signal to the ECM. P2559 sets when that signal rises above its expected range, often because of a short to reference voltage or an open signal return, so a low-coolant condition may go unreported.
Common causes
Most likely first.
- Signal wire shorted to reference voltage
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
Symptoms
- Check-engine light or coolant-level warning, if equipped
- Low-coolant warning may be absent or inaccurate
How to diagnose it
- With the engine cold, check the expansion-tank level and save freeze-frame data before clearing the code.
- Review live coolant-level data, if available, and compare it with the visible coolant level and warning status.
- Inspect the sensor connector and wiring for loose or corroded terminals, damaged return wiring, or a signal wire pinched against a voltage supply.
- Move the harness gently while watching live data to check for an intermittent open or short that makes the signal rise.
- Use the wiring diagram to test for a short to reference voltage and verify the signal return and sensor circuit to the manufacturer's specification.
Common fixes
- Repair a signal wire shorted to a voltage source.
- Repair an open or damaged coolant-level sensor return wire.
- Clean and secure corroded or loose connector terminals.
- Replace the level sensor or switch if it fails its specified test with sound wiring.
Can I keep driving?
Check the coolant level manually with the engine cold because this fault can make a low-coolant warning absent or inaccurate. Stop driving if the engine overheats or coolant is low to avoid 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.