P0666 — PCM/ECM/TCM Internal Temperature Sensor “A” Circuit #

Note: The wording of this code may vary depending on the vehicle manufacturer.

• P0666: Powertrain Control Module (PCM) Internal Temperature Sensor “A” Circuit (General, most OEMs)
• P0666: ECM Internal Temp Sensor “A” Circuit (Toyota, Lexus, Honda)
• P0666: TCM Internal Temperature Sensor “A” Circuit (Nissan, FCA/Chrysler)
• P0666: Internal Control Module Temperature Sensor A Circuit (Ford, GM, Hyundai/Kia)

Summary of code P0666 #

The P0666 diagnostic trouble code (DTC) indicates a detected issue with the internal temperature sensor circuit labeled “A” inside the Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM). These modules are the onboard computers that manage engine and transmission performance. They utilize internal temperature sensors to monitor their own operating conditions, ensuring safe and efficient operation. When the circuit for sensor “A” is out of range, shorted, open, or otherwise abnormal, the module sets this code. While the code may sound obscure, it can have real impacts on drivability, module lifespan, and overall system performance.

Common symptoms for code P0666 #

A vehicle with a P0666 code may not always show dramatic symptoms, but ignoring it can risk further module damage. Depending on the OEM and severity, you might notice:

• Check Engine Light (CEL) or Service Engine Soon warning illuminated
• Intermittent or hard starting
• Engine or transmission performance issues (reduced power, limp mode)
• Unexpected fan operation or cooling system warnings
• Possible stalling or surging
• No symptoms at all, in rare cases

Probable causes for code P0666 (ranked by likelihood) #

The root causes for a P0666 code typically stem from issues inside the control module or its internal sensor. Here’s what you’re most likely facing, in order of frequency:

1. Internal sensor failure: The temperature sensor inside the PCM/ECM/TCM has degraded or failed.
2. Circuit board fault: A damaged or corroded circuit board trace inside the module.
3. Module overheating: External heat or electrical loads causing the module to overheat and misread temperature.
4. Voltage supply issue: Poor power or ground to the module affecting internal sensor readings.
5. Software/firmware glitch: Rare, but possible if the OEM has released updates for module logic.

Step-by-step diagnosis for code P0666 #

A careful, methodical approach is essential for diagnosing P0666, since internal sensor issues often require module replacement or specialist repair. Here’s a proven process:

• 1. Record the code and freeze-frame data using a scan tool. Note when and how often the code is set.
• 2. Check for additional codes (especially voltage, ground, or communication errors) that could point to related problems.
• 3. Inspect module mounting and location. Ensure the PCM/ECM/TCM is not exposed to excess heat (near exhaust, aftermarket equipment, or engine leaks).
• 4. Test battery voltage and ground at the module’s connectors. Low voltage or poor ground can skew internal sensor readings.
• 5. Use a scan tool to monitor the reported internal module temperature. Compare with ambient and underhood temperatures. If readings are wildly inaccurate or illogical, suspect an internal fault.
• 6. Check for technical service bulletins (TSBs) from the OEM regarding P0666 or module sensor issues.
• 7. If all wiring, voltage, and mounting checks out, suspect an internal failure—the module may need to be repaired or replaced.

Possible repairs for code P0666 #

The repair for P0666 depends on what your diagnosis uncovers. Here are the most common fixes tied to the likely causes:

• Replace the PCM/ECM/TCM if the internal temperature sensor is faulty or readings remain abnormal
• Repair or clean module mounting if excess heat or contamination is present
• Restore proper power and ground at the module connector if voltage is too low or ground is weak
• Install heat shielding or relocate the module if it’s being overheated by nearby components
• Update module software if a TSB or recall applies for your vehicle

Is it safe to drive with code P0666? #

Limit driving until repaired. While some vehicles may run normally with a stored P0666 code, others can enter limp mode or experience severe drivability issues without warning. More importantly, continued operation with an overheating module or faulty internal sensor can quickly lead to permanent module failure—a much costlier repair. It’s best to have the issue diagnosed and fixed as soon as possible.

Vehicles most commonly affected by code P0666 #

This code appears across a wide range of vehicles, but is most frequently reported on certain models from:

• Ford (F-series trucks, Focus, Fusion)
• Chevrolet (Silverado, Malibu, Cruze)
• Toyota (Camry, Corolla, Tacoma)
• Honda (Civic, Accord, CR-V)
• Nissan (Altima, Sentra, Rogue)
• Hyundai/Kia (Elantra, Sonata, Sorento)
• Chrysler/Jeep/Dodge (Ram, Grand Cherokee, Charger)

However, P0666 can occur in almost any modern car, truck, or SUV equipped with advanced electronic control modules.

Common mistakes & best practices for code P0666 #

Because P0666 is an internal module issue, misdiagnosis and “shotgun” parts replacement are common. Here’s what to avoid—and what to do instead:

• Don’t replace sensors or wiring outside the module—the fault is almost always inside the PCM/ECM/TCM itself.
• Don’t ignore voltage or ground issues—low system voltage can mimic internal sensor faults.
• Do check for OEM TSBs—a known software update or module replacement campaign may apply.
• Do compare module temperature readings with reality using scan data and an infrared thermometer, if possible.
• Don’t clear the code and hope for the best—it will almost always return, and underlying issues may worsen.

FAQ for code P0666 #

Q: Can I just reset the code and keep driving?
A: Clearing the code without addressing the underlying issue will not fix the problem. The code is likely to return, and you risk further damage to the module.

Q: Is module replacement the only fix?
A: In most cases, yes, if the internal sensor or circuit has failed. However, always check for voltage/ground issues and TSBs first—sometimes, a less expensive repair or software update is possible.

If you’re unsure, consult a qualified technician or your local dealership. Prompt attention to P0666 can mean the difference between a simple repair and an expensive, time-consuming module replacement.