Circuit Description
The engine control module (ECM) controls the throttle blades by applying a varying voltage to the throttle valve motor. The ECM monitors the actual throttle blade position using throttle position (TP) sensor 1 and 2. If the ECM detects the throttle blade did not return to the rest position during the learn procedure, DTC P1551 sets.
Conditions for Running the DTC
| • | The ECM is performing a throttle learn procedure. |
| • | The ignition is ON. |
| • | The vehicle speed is 0 km/h (0 mph). |
| • | The engine speed is less than 250 RPM. |
| • | The engine coolant temperature (ECT) is 5-100°C (41-212°F). |
| • | The intake air temperature (IAT) is more than 5°C (41°F). |
| • | The battery voltage is more than 10 volts. |
| • | The accelerator pedal position (APP) is less than 15 percent. |
Conditions for Setting the DTC
The control module determines that the throttle valves are not in the rest position during the throttle learn procedure.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame and/or the Failure Records. |
| • | The control module commands the TAC system to operate in the Reduced Engine Power mode. |
| • | A message center or an indicator displays Reduced Engine Power. |
| • | Under certain conditions the control module commands the engine OFF. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
| • | Inspect for a condition in which the throttle valves may have been held open. |
| • | Inspect for conditions in which ice may have formed in the throttle bore. |
| • | The throttle valves are spring loaded to a slightly open position. This is referred to as the rest position. The throttle valves should not be completely closed, nor should they be open any more than the specified amount. The throttle valves should move open and to the closed position without binding under the normal spring pressure. The throttle should NOT be free to move open or closed WITHOUT spring pressure. |
| Important: Operating the throttle blade with the Throttle Blade Control function of the scan tool may cause additional DTCs to set. DO NOT attempt to diagnose DTCs set during this function. |
| • | The scan tool has the ability to operate the throttle control system using the Throttle Position function located in the TAC system menu. This function will operate the throttle valve through the entire range in order to determine if the throttle body and system operate correctly. |
| • | For intermittent conditions, refer to Intermittent Conditions . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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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.
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When the ignition is turned 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.
Step | Action | Values | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Connector End Views Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||||
2 | Observe the DTC information with a scan tool. Are DTCs P0121, P0122, P0123, P0221, P0222, P0223 also set? | -- | Go to Step 3 | |||||||
Did you find any of the above 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.
Is the voltage within the specified range for both circuits? | 2-4 V | Go to Step 9 | Go to Step 5 | ||||||
5 | Is the voltage less than the specified value? | 2 V | Go to Step 8 | Go to Step 7 | ||||||
Does the test lamp flash ON and then turn OFF? | -- | Go to Step 9 | Go to Step 12 | |||||||
7 | Test the TAC motor control circuit that measured above the specified value for a short to voltage. Refer to 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 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 shorted and poor connections 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 Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- | ||||||
13 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 14 | ||||||
14 | Observe the Capture Info with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||
