Code
P1639
GMC
P — Powertrain
5 Volt Reference 2 Circuit
Views:
UK: 37
EN: 85
RU: 45
AI status
Completed
Completed
100%
Causes
- Open circuit in the 5 V reference wiring
- Short to ground or short to battery voltage on the 5 V reference circuit
- Damaged or corroded sensor connector(s)
- Faulty sensor(s) tied to 5 V reference 2 creating a short
- Poor PCM/ECM ground or internal PCM/ECM fault
- Aftermarket equipment or previous repairs modifying wiring harness
Symptoms
- MIL (check engine light) illuminated
- Multiple sensor-related trouble codes or erratic sensor readings
- Poor drivability (surging, rough idle, hesitation) depending on affected sensors
- Engine may stall or run poorly if key sensors lose the reference
- Possible failure to start if multiple sensors are affected
What to check
- Read freeze frame and related codes to identify which sensors share 5V reference 2
- Visual inspection of wiring and connectors on affected sensors for damage, corrosion, or water intrusion
- Backprobe the reference pin at each sensor and at the PCM with ignition ON (engine OFF) and measure voltage
- Inspect PCM/ECM connector and ground points for corrosion or looseness
- Disconnect sensors on the reference circuit one at a time to see if code clears (to locate a shorted sensor)
- Perform a wiggle test on harness while monitoring reference voltage for intermittent faults
Signal parameters
- Expected voltage: approximately 5.0 V DC (typical tolerance ±0.2–0.5 V depending on vehicle)
- Signal type: steady DC reference voltage supplied by PCM/ECM
- Supplies multiple sensors; low current source (sensors draw small currents)
- Fault states: open (no voltage), short-to-ground (0 V or low), short-to-battery (≈12 V or higher than expected)
- Measure with ignition ON, engine OFF for stable reading; some vehicles require engine running for target tolerance tests
Diagnostic algorithm
- Retrieve all stored DTCs and freeze-frame data. Note which sensors or circuits are listed alongside P1639.
- Perform a thorough visual inspection of wiring, harness routing, and connectors for sensors that share 5V reference 2.
- With ignition ON (engine OFF), backprobe the 5V reference pin at the PCM and at each affected sensor. Confirm presence of ~5.0 V at the PCM and at each sensor connector.
- If the PCM shows ~5 V but a sensor pin is low/0 V, disconnect that sensor and recheck the reference at the harness. If voltage returns with sensor disconnected, suspect that sensor is shorting the circuit.
- If the reference is low or absent at the PCM connector, inspect PCM power and ground circuits and related fuses/relays. Check for damage or corrosion at the PCM connector.
- Use a multimeter continuity/ohms check to locate shorts to ground or to battery between the reference pin and chassis/battery. For intermittent faults use a short-finder tool or low-amp current clamp.
- Repair or replace damaged wiring, connectors, or the offending sensor. Make high-quality repairs (proper soldering, heat-shrink, correct terminals).
- After repair, erase codes, perform readiness drive cycle and verify code does not return. If the fault remains and all harness and sensors test good, consider PCM replacement only after confirming power/grounds and with manufacturer verification.
Likely causes
- Short to ground in the 5 V reference wiring (most common)
- Damaged connector or wiring harness at a sensor
- Failing sensor on the 5 V reference 2 circuit
- Open circuit between PCM and sensors
- PCM internal fault (least common)
Fault status
Status
P1639 — 5 Volt Reference 2 Circuit: PCM detected out-of-range/open/short condition on the 5 V reference supply used by sensors on reference bank 2.
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
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