Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET2001CAVALIER LS, 2.4 TREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 86 (ENGINE CONTROLS SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P1639: 5-VOLT REFERENCE B CIRCUITDIAGNOSTIC PROCEDURES
2001 Chevrolet Cavalier LS, 2.4 T
Diagnostic Procedures
2001 Chevrolet Cavalier LS, 2.4 TSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2003 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
- 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.
- Observe the Freeze Frame and/or Failure Records data for this DTC. 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. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM.
- Turn ignition OFF. Disconnect the A/C pressure sensor electrical connector. Turn ignition ON, with engine OFF. Measure the voltage between the 5-volt reference circuit and engine ground. Is voltage 4.8-5.2 V? If yes, go to next step. If no, go to step 5.
- Connect the A/C pressure sensor. Disconnect the FTP sensor. Measure the voltage from the FTP sensor 5-volt reference circuit and a good ground. Is the voltage 4.8-5.2 V? If yes, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If no, go to step 11.
- Is the voltage measured in step 3 more than 5.2 V? If yes, go to step 8. If no, go to next step.
- Monitor the voltage while disconnecting the FTP sensor. Does the voltage return to 4.8-5.2 V when the FTP is disconnected? If yes, go to step 10. If no, go to next step.
- Turn ignition OFF. Disconnect the PCM. Test the 5-volt reference circuit for a short to ground or any sensor low reference circuit. Did you find and correct the condition? If yes, go to step 13. If no, go to step 12.
- Turn ignition OFF. Disconnect PCM. Turn ignition ON, with engine OFF. Test all 5-volt reference circuits for a short to voltage. Did you find and correct the condition? If yes, go to step 13. If no, go to next step.
- Test the FTP sensor signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step 13. If no, go to step 12.
- Replace the FTP sensor. See FUEL TANK PRESSURE SENSOR under SENSORS in REMOVAL & INSTALLATION - CORVETTE article. Did you complete the replacement? If yes, go to step 13.
- Replace the A/C pressure sensor. See appropriate article in HVAC. Did you complete the replacement? If yes, go to step 13.
- Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL & INSTALLATION - CORVETTE article. Did you complete the replacement? If yes, go to next step.
- 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 cycle? If yes, go to step 2. If no, go to next step.
- Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.