What does it mean?
P1550 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Stepper Motor Cruise Control Circuit Malfunction .
Symptoms
- Cruise control cannot be switched on, or drops out shortly after being set
- Cruise indicator lamp flashes or refuses to light
- Set speed hunts, surges or falls away on gentle gradients
- Check engine light on with the code stored
- The vehicle otherwise drives normally, which points to a control circuit rather than an engine fault
Likely causes
- Open or shorted winding in one of the stepper motor phases inside the cruise servo
- Corroded or water-damaged connector at the cruise control actuator
- Chafed or broken wiring between the cruise control module and the servo, often where the harness crosses the bulkhead
- Poor ground or missing supply voltage at the actuator caused by a blown fuse
- Worn drive gear or a seized actuator shaft that overloads the stepper motor
- Failed driver circuit in the cruise control or engine control module
How to fix it
- Confirm the basic enable conditions first: cruise main switch, brake switch and a valid vehicle speed signal
- Inspect the servo connector for corrosion, bent pins and moisture, then reseat it
- Measure the resistance of each stepper motor phase and compare the readings; one deviating winding identifies a failed motor
- Check for supply voltage and a clean ground at the actuator with the ignition on
- Wiggle-test the harness along the bulkhead while monitoring the circuit for an intermittent break
- Check that the actuator shaft and linkage move freely and are not seized or binding
- Replace the servo assembly if a winding is out of specification, and consider the control module only afterwards