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Center High Mounted Stop Lamp Malfunction

WARNING: This page is about a different car, the 2010 Pontiac G5 and 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ignition OFF, disconnect the CHMSL relay.
  2. Ignition OFF, exterior lamps OFF, test for less than 5 Ω between the ground circuit terminal 86 and ground.
    • If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.
    • If the test lamp illuminates, test the control circuit for a short to voltage.
  4. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.
    • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal 87 for a short to ground. If the circuit tests normal, test or replace the CHMSL relay.
  5. Ignition OFF, disconnect the harness connector at the CHMSL.
  6. Ignition OFF, exterior lamps OFF, test for less than 5 Ω between the ground circuit terminal B and ground.
    • If greater than the specified range, test the ground circuit for an open/high resistance.
  7. Connect the harness connector at the CHMSL.
  8. Connect a 10 A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the CHMSL is illuminated.
    • If the CHMSL does not illuminate, test the control circuit for an open/high resistance. If the circuit tests normal, replace the CHMSL.
  9. Connect a test lamp between the control circuit terminal 85 and ground.
  10. Ignition ON, turn the stop lamps ON and OFF by applying and releasing the brake pedal. The test lamp should turn ON and OFF when changing between the selected states.
    • If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.
    • If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.
  11. If all circuits test normal, replace the CHMSL relay.
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