Circuit Description
The Throttle Position (TP) sensor provides a voltage signal that changes relative to the throttle opening. The TP sensor signal voltage varies from about 0.6 volt at idle to about 4.8 volts at wide open throttle (WOT).
The TP sensor signal is an important input used by the ECU for fuel trim and diagnostic functions.
This is a type B DTC.
Conditions for Setting the DTC
The following conditions will set the DTC:
| • | The vehicle is in the CNG operation mode. |
| • | The TP sensor circuit voltage is greater than 4.9 volts. |
Action Taken When the DTC Sets
The ECU sets the DTC 21 after 2 consecutive driving trips with the fault active.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after 3 consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
A scan tool displays the throttle position in volts. It should display 0.40 volt to 0.90 volt with the throttle closed and the ignition ON or at idle. The voltage should increase at a steady rate as the throttle is moved toward Wide Open Throttle (WOT). The scan tool also displays the throttle angle percentage where 0% = closed and 100% = WOT.
Scan the TP sensor signal while depressing the accelerator pedal with the engine stopped and the ignition ON. The display should vary from below 1.25 volts when the throttle is closed to over 4.5 volts when the throttle is held in the Wide Open Throttle (WOT) position.
This DTC will result if the TP sensor ground circuit is open or the TP sensor signal circuit is shorted to the voltage.
Refer to Intermittent Conditions if the DTC is intermittent.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
-
A fault exists if the TP sensor signal measures greater than 4.9 volts.
-
With the ECU connector disconnected and the voltage still present, this eliminates the ECU as the possible cause of the fault.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
Does the scan tool display the TP sensor signal greater than the specified value? | 4.8 V | Go to Step 3 | Go to Step 7 | |
3 | Backprobe the TP sensor signal in the ECU connector using a DVOM. Does the DVOM display the TP sensor signal greater than the specified value? | 4.8 V | Go to Step 4 | Go to Step 7 |
Disconnect ECU connector C001. Does the DVOM display the TP sensor signal greater than the specified value? | 4.8 V | Go to Step 6 | Go to Step 5 | |
5 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 21 | -- |
6 |
Does the scan tool display a TP sensor signal less than the specified value? | 0.2 V | Go to Step 8 | Go to Step 11 |
7 | The DTC is intermittent. Refer to Diagnostic Aids if no additional DTCs are stored. If additional DTCs are stored, refer to those tables first. Are any additional DTCs stored? | -- | Go to the applicable DTC table | -- |
8 | Probe the 5 volt reference circuit at the TP sensor electrical connector with a DVOM connected to ground. Is the voltage greater than the specified value? | 5.2 V | Go to Step 12 | Go to Step 9 |
9 | Probe the TP sensor ground circuit at the TP sensor connector with a test light connected to B+. Is the test lamp ON? | -- | Go to Step 10 | Go to Step 16 |
10 | Replace the TP sensor. Refer to TP Sensor Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 21 | -- |
11 | Inspect for a short to voltage in the TP sensor signal circuit. Was a problem found? | -- | Go to Step 17 | Go to Step 20 |
12 |
Is the voltage greater than the specified value? | 5.2 V | Go to Step 15 | Go to Step 13 |
13 | Measure the voltage on the VCM connector terminal CLR27 with a DVOM connected to ground. Is the voltage greater than the specified value? | 5.2 V | Go to Step 14 | Go to Step 20 |
14 |
Is the voltage greater than the specified value? | 5.2 V | Go to Step 19 | Go to Step 18 |
15 | Repair the short to voltage on the 5 volt reference circuit. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 21 | -- |
16 | Inspect the sensor ground circuit for an open. Was a problem found? | -- | Go to Step 17 | Go to Step 20 |
17 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 21 | -- |
18 | Replace the EGR valve. Refer to EGR Valve Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 21 | -- |
19 | Repair the short to voltage on the 5 volt reference circuit terminal CLR27. Is the action complete? | -- | Go to Step 21 | -- |
20 | Replace the VCM. Important:: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Programming in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 21 | -- |
21 |
Does the diagnostic run and pass? | -- | Go to Step 22 | Go to Step 2 |
22 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
