What the P0577 code means
The PCM reads an electrical fault in the cruise control circuit.
What causes a P0577 code?
The PCM and cruise control module work together to control vehicle speed. If the PCM detects a fault in this circuit, it initiates a self-test of the entire cruise control system. This code will be stored if the PCM reads an abnormal voltage/resistance level in the input circuit from the cruise control. P0577 codes are commonly due to a failed cruise control switch. A short or open circuit in the cruise control buttons can easily be due to a spill of liquid on the switches.
What are the symptoms of a P0577 code?
There may be no drivability symptoms other than an illuminated MIL and a non-functioning cruise control. A blown fuse may also accompany the P0577 code.
How does a mechanic diagnose a P0577 code?
When cruise control is activated, the PCM uses vehicle speed sensor data to help regulate vehicle speed. In most cases, speed and throttle position are regulated by a small servo motor; as the vehicle climbs a hill or the driver presses the “accel” button, the servo opens the throttle plate, accelerating the vehicle and raising engine RPMs. As the vehicle decelerates or descends a hill, the servo closes the throttle plate, slowing the vehicle and engine RPMs.
On most older vehicles, a plastic-covered steel cable connects the throttle plate to the servo. Other newer vehicles use a drive-by-wire system, which is a bit of a misnomer since there is no accelerator cable in drive-by-wire. These vehicles still use the same actuator motor setup to open and close the throttle plate, however.
The technician will need an OBD-II scanner/code reader and digital volt/ohmmeter for a successful diagnosis.
Begin with a visual check of all wiring and connectors and replace/repair any damaged, disconnected, shorted or corroded wirring, connectors and components if needed.
Retest the system to see if repairs were successful. If wiring, connectors and components (including fuses) seem to be in good working order, connect the scanner to the diagnostic port. Record any stored codes and freeze frame data.
Clear the codes and test drive the vehicle to see if codes return. If the code fails to immediately return, there may be an intermittent condition which may need to be allowed to worsen and return in order to get an accurate diagnosis.
If all system parts are in good working order, move on to the cruise control switch itself. Replace the switch and test drive the vehicle, using the cruise control system at length to see if the problem returns. If the code returns or the cruise control doesn’t seem to work properly, raise the vehicle’s drive wheels (safely securing the vehicle).
With a helper inside the vehicle, place the vehicle in drive and let the speedometer reach 25-35 mph, then set the cruise control.
Disconnect the cruise control switch’s electrical connector and test for voltage, using the digital volt/ohmmeter. Compare readings with manufacturer’s specs.
If there is no ground signal or voltage at the switch, disconnect the connectors for the PCM and cruise control module and do a continuity test on circuits between the switches, the correct fuse panel and the PCM.
Compare your readings to factory specs and repair any open or shorted circuits. Clear any codes and retest the system.
If the continuity test reveals no problems, use the digital volt/ohmmeter to perform a resistance test on the cruise control on/off switch and buttons. Replace switches as needed. Clear codes and retest the vehicle.
Common mistakes while diagnosing P0577 code
Since a P0577 code usually involves a blown fuse, technicians often use the blown fuse as a starting point and unnecessarily replace components and parts due to the blown fuse.
How serious is a P0577 code?
A P0577 code means that the vehicle’s cruise control will be inoperable, but usually presents no other problems.
What repairs can fix a P0577 code?
Addition comments for consideration for P0577 code
Remember to leave the PCM and cruise control module disconnected while testing circuit resistance.
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