Diagnostic Aids

WARNING: This page is about a different car, the 2007 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.

IMPORTANT: Be sure to verify that the ECM engine grounds are secure and clean.

If DTC P0645 cannot be duplicated, reviewing the Failure Records vehicle millage since the diagnostic test last failed may help determine how often the condition that caused the DTC to set occurs. This may assist in diagnosing the condition.

If the condition is not present, refer to Testing for Intermittent Conditions and Poor Connections .

DTC P0645

Step	Action	Yes	No
Schematic Reference: HVAC Schematics Connector End View Reference: HVAC Connector End Views
1	Did you perform the Diagnostic System Check - Vehicle?	Go to Step 2	Go to Diagnostic System Check - Vehicle
2	Install a scan tool. Turn ON the ignition, with the engine OFF. With a scan tool, command the A/C Relay ON and OFF in the engine control module (ECM) Special Functions, Engine Output Controls list. Does the A/C Relay turn ON and OFF with each command?	Go to Testing for Intermittent Conditions and Poor Connections	Go to Step 3
3	Turn OFF the ignition. Disconnect the A/C compressor clutch relay. Turn ON the ignition, with the engine OFF. Probe the battery positive voltage circuit of the A/C compressor clutch relay with a test lamp that is connected to a good ground. Does the test lamp illuminate?	Go to Step 4	Go to Step 8
4	Connect a test lamp between the control circuit of the A/C compressor clutch relay and the battery positive voltage circuit of the A/C compressor clutch relay. Start the engine. With a scan tool, command the A/C Relay ON and OFF. Does the test lamp turn ON and OFF with each command?	Go to Step 6	Go to Step 5
5	Test the control circuit of the A/C compressor clutch relay for a short to ground, short to voltage, or an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition?	Go to Step 11	Go to Step 7
6	Inspect for poor connections at the A/C compressor clutch relay. Refer to Testing for Intermittent Conditions and Poor Connections andConnector Repairs . Did you find and correct the condition?	Go to Step 11	Go to Step 9
7	Inspect for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition?	Go to Step 11	Go to Step 10
8	Repair the battery positive voltage circuit of the A/C compressor clutch relay. Refer to Wiring Repairs . Did you complete the repair?	Go to Step 11	-
9	Replace the A/C compressor clutch relay. Did you complete the replacement?	Go to Step 11	-
10	Replace the ECM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement?	Go to Step 11	-
11	Use the scan tool in order to clear the DTCs. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text. Does the DTC reset?	Go to Step 2	System OK