P1605

Symptoms

• Check engine light
• rough or unstable idle — RPM bounces or feels shaky
• especially in the first minute after startup
• engine may stall at idle or when shifting into gear
• reduced power and acceleration
• increased fuel consumption
• knocking or pinging sounds in severe cases
• especially under load. Often appears alongside P1603 (Engine Stalling) and P1604 (Startability Malfunction).

Probable causes

• On 1996-2002 models (knock control CPU meaning): hardware failure of the knock control CPU inside the ECM — almost always requires ECM replacement or repair
• malfunctioning knock sensors or their connectors providing bad input can also trigger the code. On 2003+ models (rough idling meaning): a vacuum leak
• dirty MAF sensor
• or ignition issue in the vast majority of cases.

Diagnostic steps

1. Confirm the model year and engine to establish which P1605 definition applies.
2. Check for companion codes (P1603, P1604) and record freeze frame data.
3. Inspect the knock sensor wiring and connectors for damage or corrosion.
4. On 2003+ vehicles (rough idling definition): check for vacuum leaks, inspect/clean the MAF sensor, and verify the ignition system before anything else.
5. On 1996-2002 vehicles (knock control CPU definition): if knock sensors and wiring test good, the fault is internal to the ECM — repair or replace the ECM.
6. Clear the code and confirm the fix with a road test under load.