P0666
PCM / ECM / TCM Internal Temperature Sensor Circuit
Powertrain Network/Communication ECU Internal Sensor 🟡 Moderate — Fix within a week
💬
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.

⚡
Symptoms You May Notice
3 known symptoms for this code
Check Engine Light illuminated
Reduced engine performance or limp mode
Possible transmission shifting issues
🔬
Embedded Systems Insight
What the ECU/ECM is actually computing

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

Parameter Normal Range Fault 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
🔧
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.