P0666
Control Module Internal Temperature Sensor A Circuit
Computer / control module
Generic OBD-II
High
What it means
P0666 means the internal temperature sensor inside the powertrain, engine, or transmission control module is reading outside its normal range. The module watches its own temperature so it can protect itself from heat; when that reading looks wrong, it sets this code and may reduce power as a precaution. It usually points at the sensor, module wiring, or the module itself rather than at the engine.
High
Can I keep driving? Usually driveable, but the module may reduce power to protect itself if it thinks it is overheating — get it diagnosed rather than ignore it.
Common symptoms
What you'll notice
- Check-engine light on
- Reduced power or limp mode if the module derates to protect itself
- Possible intermittent stalling or no-restart when the module is hot
- Other module or driveability codes stored alongside this one
Common causes
Most likely first
- Faulty internal temperature sensor inside the PCM/ECM/TCMLikely
- Poor grounds or corroded module connectors giving a false readingCommon
- Actual module overheating (blocked airflow, water intrusion, heat soak)Common
- Failed control module (PCM/ECM/TCM)Less often
How to diagnose & fix
Work through it in order
- Scan and record freeze-frame data; note any companion codes that point at the same module.
- Inspect the module's power, ground, and connector pins for corrosion or looseness and clean/repair as needed.
- Check the module's mounting and surroundings for real overheating — water intrusion, blocked airflow, or nearby heat sources.
- If the internal sensor reading stays out of range with good wiring and no real overheating, the control module itself is at fault and needs replacement/reprogramming.
FAQ
Quick answers
Is it safe to drive with P0666?+
Short trips are usually fine, but if the module thinks it is overheating it can cut power without warning. Have it checked before relying on the vehicle.
Can I just clear P0666?+
It will clear, but it returns if the internal sensor or its wiring is truly faulty. This is an internal-module reading, so diagnosis usually needs a scan tool and connector inspection.
Related codes
Often seen with P0666
Read & clear P0666 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX