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 MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 85 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 2.2L)DIAGNOSTIC TESTSDTC P1639: 5-VOLT REFERENCE 2 CIRCUITDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
- Turn ignition off for 10 seconds. Turn ignition on, engine off. Wait 10 seconds. Using scan tool, monitor status of DTC P1639 LAST TEST. If LAST TEST failed, go to next step. If LAST TEST did not fail, problem is intermittent. See DIAGNOSTIC AIDS .
- Disconnect one 5-volt reference sensor at a time (TP2, APP1, and MAP) while using scan tool to monitor DTC P1639 LAST TEST. Leave each sensor disconnected. If LAST TEST passed when any one component is disconnected, replace component if test passes when the TP2, APP1 or MAP sensor connector is disconnected. If test passes when fuel tank in-line connector is disconnected, check for short to ground in Gray wire between fuel tank in-line connector and FTP sensor. If wire is okay, replace FTP sensor. If LAST TEST does not pass when any one component is disconnected, go to next step.
- Using DVOM, measure voltage on TP sensor 2 connector terminal "D" (Purple wire). If voltage is less than 4.5 volts, 5-volt reference 2 circuit is shorted to ground. To locate short to ground, turn ignition off. Disconnect ECM connectors J1 and J2. Check continuity between 5-volt reference circuit and ground, then between 5-volt reference circuit and ground circuit at each sensor harness connector. If continuity on any circuit exists, repair short in that circuit. If continuity does not exist in any test, replace ECM. If voltage is not less than 4.5 volts, 5-volt reference 2 circuit is shorted to another ECM voltage source. Disconnect ECM connectors J1 and J2. Check continuity between 5-volt reference 2 circuits and all other ECM circuits. Repair short between circuits where continuity exists.
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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.