P0666

PCM / ECM / TCM Internal Temperature Sensor Circuit

Powertrain Network/Communication ECU Internal Sensor 🟡 Moderate — Fix within a week ⚠️ Drive with Care
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What This Actually Means

In plain language — no jargon

The engine computer has an internal temperature sensor that monitors itself, and it's detecting a problem with that sensor's electrical circuit. Think of it like a thermometer inside the computer that's broken or disconnected.

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Symptoms You May Notice

3 known symptoms for this code
Check Engine Light illuminated
Reduced engine performance or limp mode
Possible transmission shifting issues
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The PCM/ECM monitors its own internal die temperature using an embedded thermistor to prevent overheating and protect circuits. The sensor circuit is continuously checked for open circuits, shorts, or out-of-range voltage signals. If resistance or voltage falls outside expected parameters, the fault is triggered.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
Internal temp sensor voltage 0.5–4.5V at typical operating temp Below 0.1V or above 4.9V, or no signal
PCM die temperature -40°C to +125°C range Sensor circuit open or shorted; out-of-range reading
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Battery terminals and connectors
Clean corrosion from battery posts and check all ECU ground straps for loose or corroded connections.
2
ECU connectors
Inspect and reseat the main ECU harness connectors under the dash or hood; look for bent pins or moisture.
3
Engine Control Module (ECM)
If voltage and connectors test good, the ECU itself likely has a failed internal temperature sensor and requires replacement or reprogramming by a dealer.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code P0666 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code P0666

What happens after you fix the fault

Once the fault is repaired, P0666 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.