What does it mean?
P1666 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cruise Control Module Internal .
Symptoms
- MIL on and the cruise control will not engage at all
- Cruise engages but drops out on its own after a short time
- Set speed is not held: the vehicle creeps up or falls back on level road
- The cruise indicator behaves oddly, flashing or staying lit
- Scan tool cannot communicate with the cruise control module
Likely causes
- Genuine internal fault in the cruise control module electronics
- Water or corrosion inside the module connector or housing
- Poor power feed or ground at the module, or a blown supply fuse
- Low system voltage from a weak battery or a failing alternator
- Faulty brake or clutch switch input confusing the module logic
- Voltage spike from a jump start or from welding on the vehicle
- Interrupted software update on modules that are programmable
How to fix it
- Measure battery and charging voltage first, since supply problems mimic an internal module fault
- Check the module fuse and measure supply voltage and ground quality at the module connector
- Unplug the connector and inspect for water intrusion, corrosion and pushed-out terminals
- Watch the brake and clutch switch states on live data while pressing each pedal; a stuck switch prevents engagement and can flag the module
- Verify the cruise control switch inputs read correctly as each button is pressed
- Clear the code and retest over a full drive cycle to separate an active fault from a stored one
- Replace and, where required, code the cruise control module only if it still fails with good supply and correct inputs