Test Description

WARNING: This page is about a different variant/trim than selected.

The numbers below refer to the step numbers on the diagnostic table.

3: The throttle valves are spring loaded in a slightly open position and should move in either direction without binding. The throttle valves should always be under spring pressure.
6: When the ignition is turn ON, the ECM operates the throttle control motor to verify the integrity of the system prior to start-up. This can be seen by the momentary flash of the test lamp as the ignition is turned ON.

DTC P0638

Step	Action	Value	Yes	No
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module (ECM) Connector End Views or Engine Controls Connector End Views
1	Did you perform the Diagnostic System Check-Engine Controls?	-	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Observe the DTC information. Are DTCs P0121, P0122, P0123, P0221, P0222, P0223 also set?	-	Go to Diagnostic Trouble Code (DTC) List	Go to Step 3
3	Turn OFF the ignition. Visually inspect the throttle body for the following conditions: Throttle valves that are NOT in the rest position Throttle valves that are binding open or closed Throttle valves that are free to move open or closed WITHOUT spring pressure Did you find any of these conditions with the throttle body?	-	Go to Step 11	Go to Step 4
4	IMPORTANT: Disconnecting the throttle body connector will cause additional codes to set. Disconnect the throttle body harness connector. Turn ON the ignition, with the engine OFF. Measure the voltage of the TAC motor control 1 and TAC motor control 2 circuits of the throttle actuator motor with a DMM connected to ground. Is the voltage within the specified range for both circuits?	2-4 V	Go to Step 6	Go to Step 5
5	Is the voltage less than the specified value for either circuit?	2 V	Go to Step 8	Go to Step 7
6	Turn OFF the ignition. Connect the test lamp between the TAC motor control 1 and TAC motor control 2 circuit of the throttle body. Observe the test lamp as you turn ON the ignition. Does the test lamp flash ON, then turn OFF?	-	Go to Step 9	Go to Step 10
7	Test the TAC motor control circuit that measured above the specified value for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 13	Go to Step 10
8	Test the TAC motor control circuit that measured below the specified value for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 13	Go to Step 10
9	Test for an intermittent and for a poor connection at the throttle body connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 13	Go to Step 11
10	Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 13	Go to Step 12
11	Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement . Did you complete the replacement?	-	Go to Step 13	-
12	Replace the ECM. Refer to Engine Control Module (ECM) Replacement . Did you complete the replacement?	-	Go to Step 13	-
13	Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition?	-	Go to Step 2	Go to Step 14
14	Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed?	-	Go to Diagnostic Trouble Code (DTC) List	System OK