Important:  If it is found that one or more of the active fuel management (AFM) lifters are not operating properly and needs replacing, replace all 6 AFM lifters and the lifter guide as an assembly.

The Cylinder Deactivation system compression test is a functional test of the valve lifter oil manifold and valve lifters. Using a compression gage, the technician can monitor the specific cylinders ability to go into and out of Cylinder Deactivation as commanded by the scan tool. The actual pressure values on the compression gage are not as critical as is the on-off switching action of the solenoid and valve lifters.

Important: Proper oil viscosity is critical to Cylinder Deactivation Operation. ALWAYS verify proper engine oil viscosity and oil change interval BEFORE beginning AFM system diagnosis. Improper viscosity or excessively dirty engine oil may cause improper operation. Under NO circumstances should engine oil additives be used, unless recommended by General Motors. If you suspect the engine oil is dirty, contaminated, or of improper viscosity, change engine oil and filter prior to performing any Cylinder Deactivation diagnostics.

1. Check the engine oil level. Fill as required.
2. Perform the Diagnostic System Check. Refer to Diagnostic System Check - Vehicle .
3. Disable the ignition system for the cylinder to be tested, by disconnecting the spark plug wire and grounding it with ST-125 to prevent coil damage.
4. Remove the spark plug and wire for the cylinder to be tested.
5. Install a 0-1378 kPa (0-200 psi) compression gage.
6. Start the engine.
7. Using the scan tool output controls, command the solenoid ON, for the cylinder to be deactivated.
8. Depress the Schrader valve on the compression gage in order to release the pressure and zero the gage. With the engine running and the cylinder in Cylinder Deactivation mode, the compression reading should be less than 172 kPa (25 psi).
9. Using the scan tool, de-energize the solenoid while observing the reading on the compression gage. With the engine running and the cylinder NOT in Cylinder Deactivation mode, the compression gage reading should increase quickly to 482 kPa (75 psi) or higher.

Important: Only one cylinder can be tested at a time. When testing has been completed on a specific cylinder, the wire harness electrical connectors, spark plug, and spark plug wire must be installed prior to testing of each additional cylinder.

10. Test each of the three Cylinder Deactivation cylinders one at a time. If the readings are not within specifications, refer to Cylinder Deactivation (Active Fuel Management) Valve Lifter Oil Manifold Diagnosis and Testing .
11. If the compression test has been performed and all Cylinder Deactivation cylinders are operating within specifications, the Cylinder Deactivation system is performing as designed and no repairs are required.
12. Clear the DTCs with a scan tool.