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P2559Engine Coolant Level Sensor/Switch Circuit High

The engine coolant level sensor circuit is reading above its maximum expected voltage, indicating an open circuit or short to voltage.

Definition

The PCM or body module monitors the coolant level sensor circuit and sets P2559 when the input is pegged high — typically because the circuit is open (unplugged, broken wire, failed sensor) or shorted to voltage. Depending on the circuit design, the driver may see a constant low-coolant warning or no warning capability at all, and a MIL may set. The engine runs normally, but the safety net that warns of coolant loss before an overheat is compromised while the fault is present.

Common causes

  • Disconnected or failed coolant level sensor
  • Open in the sensor signal or ground wire
  • Corroded terminals at the reservoir connector (often from coolant seepage)
  • Signal wire shorted to voltage
  • Damaged wiring from front-end collision repair or reservoir replacement

Common misdiagnoses

  • Replacing the coolant reservoir when only the connector is corroded
  • ECT sensor replacement
  • Thermostat replacement for a warning-lamp complaint
  • Module replacement before checking the wiring

Troubleshooting steps

  1. 1.Verify the sensor is connected

    Confirm the reservoir sensor connector is seated — this circuit is commonly left unplugged after coolant services or reservoir swaps.

  2. 2.Inspect the connector for corrosion

    Look for green/white corrosion and coolant residue on the terminals; seeping reservoirs frequently corrode this connector open.

  3. 3.Jumper the circuit at the connector

    With key on, bridge the connector terminals (for a two-wire switch type) and confirm the module input toggles state; if it does, the wiring is good and the sensor is at fault.

  4. 4.Check continuity and for shorts to voltage

    Ohm the signal and ground wires end to end (under 1 ohm each) and confirm the signal wire does not carry battery voltage with the sensor unplugged.

  5. 5.Replace the sensor and confirm

    Install the correct OEM-style sensor, verify the level warning operates by testing at a low simulated level, then clear the code.

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