P0666
UniversalPCM / ECM / TCM Internal Temperature Sensor Circuit
Medium SeverityThis code indicates an electrical issue within the internal temperature sensor circuit of the vehicle's Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM). Such a fault suggests the control module may not be accurately monitoring or regulating its own operating temperature, potentially leading to performance degradation or premature module failure.
⚠️ Before You Begin (Safety First)
Always disconnect the vehicle's battery before attempting any electrical inspections or repairs to prevent accidental short circuits or damage to sensitive electronic components. Be extremely cautious when handling control modules, as they are costly and can be easily damaged by static electricity or incorrect handling.
Common Symptoms
- Check Engine Light illuminated
- No noticeable driveability issues (initially)
- Intermittent module performance problems
- Vehicle may enter 'limp mode' if the module begins to overheat significantly (less common but possible)
Common Causes
- Faulty internal temperature sensor within the PCM/ECM/TCM itself
- An open or short circuit within the internal sensor's wiring or circuit board traces
- Corroded or damaged pins in the control module's electrical connectors
- Internal failure of the PCM/ECM/TCM causing incorrect temperature readings
- Power supply or ground issues affecting the control module
Frequently reported on: General/Universal
Diagnostic Steps
- Use an OBD-II scanner to confirm the P0666 code and check for any related codes.
- Visually inspect the wiring harnesses and connectors leading to the PCM, ECM, and TCM for any obvious signs of damage, corrosion, or loose connections. Ensure connectors are securely seated.
- Clear the code with the scan tool and test drive the vehicle under various conditions to see if the code reappears.
- If available, use an advanced scan tool to monitor the internal temperature readings reported by the affected control module to look for erratic or implausible values.
- For more advanced troubleshooting, consult a vehicle-specific service manual for pin-out diagrams and test procedures for the control module's power and ground circuits. This typically requires a multimeter.
👨🔧 When to Call a Professional
Due to the complexity and cost associated with control modules, if basic wiring checks and diagnostics do not quickly identify a simple external fault, it is highly recommended to consult a professional automotive technician. Control module testing, repair, or replacement often requires specialized diagnostic tools and programming.
Commonly Asked Questions
What specifically does the P0666 code mean?
The P0666 code indicates a problem with the internal temperature sensor circuit located inside your vehicle's primary control modules, such as the Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM).
Is it safe to drive my car with a P0666 code?
While immediate severe symptoms might be absent, it's not advisable to drive for long periods. An unaddressed internal temperature sensor fault could lead to the control module operating at excessive temperatures, potentially causing its premature failure and more significant vehicle issues.
Can I fix the P0666 code myself, or do I need a professional?
Basic checks like inspecting wiring and clearing codes can be done by a DIYer. However, diagnosing an internal module sensor fault or replacing a control module is complex, often requiring specialized tools and professional programming, making professional assistance highly recommended.
How do I clear a P0666 code once the issue is resolved?
After addressing the potential cause, you can clear the P0666 code using an OBD-II scan tool. Disconnecting the car's battery for a few minutes can also clear codes, but the code will return if the underlying problem persists.
What is the most likely cause for P0666?
The most common cause is often an internal fault within the control module itself, specifically relating to its temperature monitoring circuitry. Less frequently, it could stem from external wiring damage affecting the module's power or ground, or issues with the module's connectors.