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Home >> GMC >> 1997 >> Forward Control 4.3 W, L35, Automatic >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 10 (Engine Controls - Tests W/Codes - 6.5L Diesel (P Series)) >> Diagnostic Trouble Codes >> DTC P0121 - Accelerator Pedal Position (App) Sensor 1 Circuit Performance >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different variant/trim than selected.
  1. Perform Powertrain ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. See the BASIC TESTING article. Then go to next step.
  2. Install scan tool. Turn ignition on, with engine off. Using scan tool, with throttle closed, observe APP voltages. APP 1 voltage should be .45-.95 volt. APP 2 voltage should be 4.0-4.5 volts. APP 3 voltage should be 3.6-4.1 volts. If voltages are as specified, go to next step. If voltages are not as specified, go to step 4).
  3. DTC is intermittent. If no additional DTCs are set, see DIAGNOSTIC AIDS. If any additional DTCs are set, go to applicable DTC test.
  4. Turn ignition off. Disconnect APP sensor connector. Turn ignition on, with engine off. Using DVOM, probe 5-volt reference circuits (terminals "D", "E" and "G") of APP sensor connectors to ground. If voltage on all circuits is equal or greater than 4.75 volts, go to next step. If voltage on all circuits is not equal or greater than 4.75 volts, go to step 6).
  5. Turn ignition on, with engine off. Using test light, probe ground circuits (terminals "A", "B" and "J") of APP sensor connectors to battery voltage. If test light illuminates on all circuits, go to step 9). If test light does not illuminate on all circuits, go to step 8).
  6. Turn ignition off. Disconnect PCM harness connector. Check for open or short to ground in 5-volt reference circuits. Repair as necessary. If open or short to ground was present in 5-volt reference circuits, go to step 11). If open or short to ground was not present in 5-volt reference circuits, go to next step.
  7. Check for poor connections at 5-volt reference terminals of PCM. Repair as necessary. If no repairs were required, go to step 11). If no terminals were replaced, go to step 10).
  8. Turn ignition off. Disconnect PCM harness connector. Check for open in ground circuits. Repair as necessary. If open was present in ground circuits, go to step 11). If open was not present in ground circuits, go to step 10).
  9. Replace APP module. After replacing module, go to step 11).
  10. Replace PCM. After replacing PCM, go to next step.
  11. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0121. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
  12. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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