Mechanics

CHEVY REDUCED ENGINE POWER LIGHT-CODE P1271

2003 Chevrolet Trailblazer

Computer problem
2003 Chevy Trailblazer 6 cyl Four Wheel Drive Automatic 175000 miles

Hi,
2003 Chevy Trailblazer-high miles well maintained. Reduced power light started coming on in January this year. Popped codes P1271 and sometimes P1275 and I replaced the accelerator position sensor. It is an intermittent problem where as it will not come on for a few days, then all of a sudden on a cold start it will come on. Shut off restart and comes on for three consecutive start cycles then fine. The TB runs fine no other issues but this code p1271 and reduced engine power light. Seems to do it primarily on a cold start. Replaced the throttle body and had GM dealer update the PCM with the newest software. Tried jiggling all of the connections to make the fault appear, but no luck. Everything looks intact. There was also a code for the camshaft position sensor and I replaced the sensor-code has not appeared for the CPS since replaced at the same time as the accelerator position sensor. Any ideas on where to look next?
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Dostrosk
May 16, 2009.



DTC P1271: ACCELERATOR PEDAL POSITION SENSORS 1 & 2 CORRELATION NOTE: To locate components, see COMPONENT LOCATIONS. For circuit reference, see WIRING DIAGRAMS. For connector terminal identification, see CONNECTOR IDENTIFICATION. Description The Accelerator Pedal Position (APP) sensors 1 and 2 are located within the accelerator pedal assembly. Each sensor has the following circuits: A 5-volt reference circuit. • A low reference circuit. • A signal circuit. • This provides the Powertrain Control Module (PCM) with a signal voltage proportional to accelerator pedal movement. The APP sensor 1 signal voltage at rest position is about the low reference and increases as the pedal is actuated. The APP sensor 2 signal voltage at rest position is about the 5-volt reference and decreases as the pedal is actuated. Conditions For Running DTC The battery voltage is more than 5.23 volts. • DTCs P1275, P1280, P1635, or P1639 not set. • The accelerator pedal is leaving the idle position. • Conditions For Setting DTC The voltage difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value. Action Taken When DTC Sets The control module illuminates the Malfunction Indicator Light (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; or, under certain conditions the control module commands the engine OFF. • The message center displays Reduced Engine Power. • Conditions For Clearing MIL/DTC PCM turns OFF the MIL after 3 consecutive drive trips that the diagnostic runs and does not fail. •A History DTC clears after 40 consecutive warm up cycles in which no failures are reported by this diagnostic or any other emission related diagnostic. • The scan tool clears the MIL/DTC. • Test Description The number below refers to the step number in the diagnostic procedures. 2 - Any circuit faults on either APP sensor 1 or 2 will set one of the DTCs listed. Refer to the appropriate test for diagnosis. Diagnostic Procedures Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. 1. Using scan tool, observe the DTC information. Is DTC P1120, P1220, P1275, P1280, P1635, or P1639 also set? If yes, see DTC P1120: THROTTLE POSITION SENSOR 1 CIRCUIT, DTC P1220: THROTTLE POSITION SENSOR 2 CIRCUIT, DTC P1275: ACCELERATOR PEDAL POSITION CIRCUIT (SENSOR 1), DTC P1280: ACCELERATOR PEDAL POSITION CIRCUIT (SENSOR 2), DTC P1635: 5-VOLT REFERENCE " A" CIRCUIT or DTC P1639: 5-VOLT REFERENCE " B" CIRCUIT. If no, go to next step. 2. Turn ignition off. Disconnect the APP sensor electrical connector. Disconnect the PCM. Use a DVOM to measure the resistance of the following circuits for each of the APP sensors: 3. Low reference circuit. • Signal circuit. • 5-volt reference circuit. • Did any of the circuits measure more than 5 ohms? If yes, go to step 5. If no, go to next step. Test for a short between any of the circuits in the APP sensor harness. Repair circuit as necessary. Did you find and correct the condition? If yes, go to step 7. If no, go to step 6. 4. Repair high resistance in the circuit. After repairs, go to step 7. 5. Replace APP sensor. See REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After repairs, go to next step. 6. Clear the DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Operate vehicle within the Conditions For Running DTC. See CONDITIONS FOR RUNNING DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2. 7. Observe the stored information, Capture Info with scan tool. Does scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS. If no, system is okay. 8. Diagnostic AidsThe PCM compares the signal of each of the accelerator pedal position sensors to each other throughout the entire range of operation. Clear the DTCs and actuate the pedal through the entire range with the ignition ON and engine OFF. Use the Connector Test Adapter Kit (J-35616-A) for any test that requires probing the PCM harness connector or a component harness connector. Using this kit will prevent damage to the harness connector terminals. If DTC is determined to be intermittent, see INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

Merlin2021
May 17, 2009.