Code
P0666
LAND ROVER
P — Powertrain
Temperature Sensor Substrate
Views:
UK: 19
EN: 24
RU: 17
AI status
Completed
Completed
100%
Causes
- Open or short in temperature sensor substrate wiring harness
- Corroded/poor connector or pin contact at sensor or ECU
- Failed temperature sensor or substrate assembly
- Poor ground or reference voltage to the sensor circuit
- Intermittent wiring damage (chafing, broken strand)
- ECM input pin fault or internal ECU fault
Symptoms
- Malfunction Indicator Lamp (MIL) / Check Engine Light illuminated
- Erratic or incorrect coolant/air/engine temperature readings on scan tool
- Engine poor warm-up, extended warm-up time, or incorrect cooling fan operation
- Reduced engine performance or entry into limp mode on some vehicles
- Possible hard starting or rough idle if ECM uses that temperature input for fueling
What to check
- Read and record freeze-frame data and all related DTCs with a professional scan tool
- Visually inspect sensor connector and wiring harness for damage, corrosion, or water intrusion
- Check for Technical Service Bulletins (TSBs) or ECU software updates for this code on the vehicle
- Backprobe sensor connector and measure reference voltage, signal voltage/resistance, and ground continuity
- Wiggle wiring while monitoring live data to detect intermittent faults
- Compare sensor readings on scan tool to ambient and expected engine temperatures
Signal parameters
- Sensor type: thermistor/temperature element (verify manufacturer spec)
- Expected signal voltage: typically ~0.2–4.5 V depending on temperature (verify spec for vehicle)
- Reference voltage: usually 5 V supply from ECM to sensor (verify)
- Expected resistance vs. temperature curve (ohms at 20 °C, 60 °C etc. per spec)
- Ground continuity: near 0 Ω between sensor ground pin and vehicle chassis/ECM ground
- ECM temperature input value vs. actual measured temperature (degrees C/F)
Diagnostic algorithm
- Connect a scan tool, record freeze-frame and related codes, note conditions when DTC set (temp, engine load, time since start)
- Clear the code and reproduce; monitor live temperature sensor data while performing a key-on and engine-run sequence
- Perform visual inspection of sensor connector and wiring for damage, corrosion, contamination, or water ingress; repair as required
- Backprobe connector with engine off: verify reference voltage present at key-on and present to sensor pin; verify good ground at sensor
- Measure sensor resistance at ambient temperature and while warming (or measure voltage while warming). Compare to manufacturer resistance/voltage vs temperature spec
- If resistance/voltage is out of spec, replace the sensor/substrate assembly and retest
- If sensor measurements are good, perform continuity checks from sensor connector to ECM pin to rule out wiring open/shorts and check for intermittent faults with wiggle tests
- If wiring and sensor are good, inspect ECM connector for corrosion and test ECM input pin; consider ECU bench test or substitute ECU if supported and necessary
- After repairs, clear codes, perform a test drive and re-scan to ensure code does not return; document repair and update ECU software if TSB indicates
Likely causes
- Damaged sensor connector—corrosion or bent pins
- Wire chafing under intake or near moving components causing intermittent open/short
- Sensor element failure (substrate/thermistor degraded)
- Missing or poor ground at sensor/ECM
- Water ingress into sensor connector causing erroneous readings
Fault status
Status
P0666 — Temperature sensor substrate circuit fault detected (implausible/intermittent/out-of-range signal).
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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