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P1187

EOT Sensor Circuit Low Voltage.

  • Saturn
  • Powertrain
  • Manufacturer code
  • Low severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
1
AreaFuel and air metering
87
FaultThe specific fault in that area

What it means

On a Saturn, P1187 means the engine control module has seen the engine oil temperature (EOT) sensor circuit voltage fall below the expected range. This usually points to a short to ground in the signal wiring or an internal sensor fault pulling the signal too low. The module then cannot read a believable oil temperature and may use a default value.

Symptoms

  • Check engine light on
  • Oil temperature reading stuck at a very high or implausible value on a scan tool
  • Slightly rough running or poor fuel economy while the module uses a default value
  • Cooling fan running more than usual on some models
  • Possible hard or erratic cold starting on some models

Possible causes

  • Faulty engine oil temperature sensor with an internal shortlikely
  • Signal wire shorted to ground, for example chafed insulation against the engine or bracketlikely
  • Damaged, corroded or water-contaminated sensor connectorpossible
  • Wiring harness damage elsewhere in the sensor circuit, such as a pinched or rubbed sectionpossible
  • Faulty engine control module input circuitrare
  • Oil contamination or oil leak affecting the sensor and its connectorrare

How to diagnose

  1. Read the stored codes and freeze frame data, noting engine temperature and conditions when the code set, then check for other temperature sensor codes.
  2. Compare the live oil temperature reading with the coolant temperature on a cold engine; they should be close. A very high or implausible oil temperature reading suggests a low-voltage fault.
  3. Inspect the sensor, connector and nearby harness for chafing, oil soaking, corrosion, heat damage or bent pins.
  4. Unplug the sensor with the ignition on and watch the live reading. If it drops to a cold or open-circuit value, the wiring and module are probably fine and the sensor is suspect.
  5. If the reading stays low with the sensor unplugged, test the signal wire for a short to ground, with the ignition off and the module disconnected where the wiring diagram requires it.
  6. Check the sensor reference and ground circuits against the wiring diagram for your model and engine.
  7. Measure the sensor resistance at a known temperature and compare it with the manufacturer specification for your model and engine.
  8. Check the oil level and condition, as the sensor location and type vary by model and engine.

How to fix

  • Replace the engine oil temperature sensor if its resistance is out of specification or it causes the fault when tested.
  • Repair or replace any chafed or shorted signal wiring and secure it clear of sharp edges and heat.
  • Clean or replace a corroded or oil-contaminated connector and terminals.
  • Repair any oil leak that is contaminating the sensor or connector.
  • Replace or repair the engine control module only after wiring and sensor faults are ruled out and the fault is confirmed.

Can I keep driving?

Low severity

The car can usually be driven while you book it in, as the module substitutes a default value. Check the oil level regularly, and have it looked at soon if running worsens or other warning lights appear.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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