Code
P1566
OLDSMOBILE
P — Powertrain
Cruise Control System Engine RPM Too High
AI status
Completed
Completed
100%
Causes
- Sticking or binding throttle (mechanical or electronic)
- Throttle body carbon buildup or dirty idle passages
- Incorrect throttle cable or cruise actuator adjustment
- Vacuum leaks affecting idle or cruise-actuator vacuum supply
- Faulty throttle position sensor (TPS) or accelerator pedal position sensor
- Faulty engine speed (crankshaft) sensor or poor RPM signal
Symptoms
- Cruise control disengages or will not maintain set speed
- Engine RPM higher than expected while cruise is on
- Unstable or surging engine speed when cruise engaged
- Check Engine Light (MIL) may be illuminated
- Vehicle may accelerate unexpectedly or fail to hold set speed
- Cruise control may not engage at all
What to check
- Use a scan tool to read freeze frame data and live engine RPM, TPS, vehicle speed and cruise status while attempting cruise
- Verify the code is current vs. historic; clear and attempt to re-create
- Visually inspect throttle body and throttle plate for carbon buildup and binding
- Inspect throttle cable, return spring and cruise actuator linkage for free movement and correct adjustment
- Inspect vacuum lines to the cruise servo (if vacuum-actuated) for leaks or deterioration
- Check condition and connections of crankshaft/camshaft position sensor(s) and wiring
Signal parameters
- Engine RPM (crank sensor): steady idle ~600–1000 rpm (varies by engine); abnormal increase during cruise indicates issue
- Throttle position sensor (TPS): typical idle 0.5–1.5 V or 4–10% (manufacturer-specific); should increase smoothly with throttle
- Vehicle speed sensor (VSS): consistent pulse/vehicle speed matching actual speed while cruise is engaged
- Cruise actuator position or vacuum (if equipped): actuator should move to maintain set speed; vacuum typically ~15–22 inHg at idle
- Intake vacuum: normal ~15–22 inHg at idle; lower vacuum may indicate leak affecting actuator control
Diagnostic algorithm
- Retrieve DTCs and freeze frame with a scan tool. Note engine RPM, vehicle speed and TPS values when the code set.
- Attempt to duplicate condition in a safe, controlled environment (level road, light traffic) while monitoring live data.
- Visually inspect throttle body and linkage. Manually operate throttle to check for binding; clean throttle body if carbon is present.
- Inspect and test throttle cable/return spring (mechanical) or throttle actuator circuitry (electronic). Repair or replace as needed.
- Check vacuum hoses and cruise actuator (if vacuum-operated) for leaks or poor connections; pressure-test vacuum system where applicable.
- Test TPS for correct voltage range, smooth change and proper reference voltage/ground. Replace if out of spec or intermittent.
- Verify crankshaft/cam sensor signals with oscilloscope or scan tool. Repair wiring or replace sensor if signal is noisy or missing.
- Inspect cruise control servo/module and related wiring/connectors. Bench-test actuator if service information allows.
- Verify brake/clutch switches are functioning and not intermittently signaling cruise cancel.
- If mechanical and sensor checks are good, check PCM/calibration and consider software updates per service bulletin before replacing major components.
- After repairs, clear codes and perform a road test to confirm normal cruise operation and that the P1566 does not return.
Likely causes
- Throttle body or throttle plate sticking (carbon build-up) causing higher idle/RPM
- Faulty or misadjusted cruise actuator or cable allowing partial throttle input
- Vacuum leak reducing system vacuum and preventing cruise actuator from closing throttle
- Faulty RPM signal (intermittent or inaccurate) from crankshaft/cam sensor
- TPS providing incorrect throttle position data to the PCM/cruise module
Fault status
Status
P1566 — Cruise Control System Engine RPM Too High: The cruise control system detected engine speed above allowed limits while cruise was active. The PCM/cruise module recorded an over‑RPM condition and set the code. Investigate mechanical throttle issues, sensor signals (RPM, TPS), cruise actuator, vacuum and wiring.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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