Code
P1639
SATURN
P — Powertrain
5 Volt Reference 2 Circuit
AI status
Completed
Completed
100%
Causes
- Open circuit in 5V reference wiring or connector
- Short to ground on the 5V reference wire
- Short to battery voltage (B+) on the 5V reference wire
- Corroded or loose sensor/ECM connector pins
- Faulty sensor drawing excessive current from the 5V reference
- Faulty PCM/ECM 5V regulator or internal circuit
Symptoms
- Check Engine Light (MIL) illuminated
- Multiple sensor-related trouble codes or intermittent codes
- Incorrect or erratic sensor readings in scan tool data (MAP, TPS, MAF, etc.)
- Poor driveability: rough idle, hesitation, stalling, reduced performance
- Some sensors may show zero volts or battery voltage instead of ~5V
What to check
- Retrieve all stored codes and freeze frame data with a scan tool; note related sensor codes
- Visually inspect wiring harnesses and connectors for damage, corrosion, pin push-out, or water intrusion
- Backprobe the 5V reference #2 pin at affected sensor connectors with key ON (engine OFF) to measure voltage
- Measure reference voltage at the PCM/ECM connector pin for the 5V reference circuit
- Wiggle harness and connectors while monitoring the reference voltage and live sensor data for intermittent faults
- Disconnect sensors one at a time (with ignition off) to see if the fault clears to isolate a shorted sensor
Signal parameters
- Nominal 5V reference: about 5.0 V (acceptable approx. 4.8–5.2 V) with key ON, engine OFF
- Short to ground: voltage near 0 V at the reference pin
- Short to battery voltage: voltage near battery voltage (~12 V) at the reference pin
- Open circuit: no voltage or intermittent voltage at sensor connector but may be present at PCM
- Expected continuity: 5V reference pin at PCM to sensor reference pin should show low resistance (wiring harness length dependent), and high resistance to ground
Diagnostic algorithm
- Confirm P1639 and note any additional codes. Use live data to identify which sensors use 5V reference #2 and observe their readings.
- Visually inspect harnesses and connectors for damage, corrosion, rodent chewing, or recent repairs near affected sensors and PCM.
- With ignition ON (engine OFF) backprobe the 5V reference #2 pin at the PCM and at each sensor that uses that reference. Record voltages and compare to nominal ~5V.
- If voltage is correct at the PCM but low/absent at a sensor, perform continuity checks and inspect connectors between PCM and that sensor to locate an open.
- If the reference at the PCM is low or high, disconnect all sensors that use that reference and test voltage again. If voltage returns to normal with sensors disconnected, suspect a shorted sensor.
- To find a short to ground, with battery disconnected, measure resistance from the 5V reference pin to chassis ground; a very low resistance indicates a short. To find short to B+, measure resistance to battery +.
- Isolate wiring sections by disconnecting connectors in-line and rechecking until the short/open is localized. Repair damaged wiring, repair or replace corroded connectors, and apply dielectric grease after repair.
- Replace any identified faulty sensor and retest. Clear codes and perform a test drive to confirm the repair.
- If wiring and sensors check good and the 5V reference is still out of range at the PCM, suspect PCM 5V regulator failure. Confirm with manufacturer-specific PCM tests before PCM replacement; replace/program PCM only if confirmed.
Likely causes
- Damaged or chafed wiring harness to one or more sensors using 5V reference #2 (most likely)
- Corroded/poor connector at a sensor or PCM (next most likely)
- Failed sensor that is shorting the reference to ground or B+
- PCM internal 5V regulator failure (least likely but possible)
Fault status
Status
PCM detected an abnormal condition on the 5 Volt Reference Circuit 2 (open, short to ground, or short to battery). Sensors using this reference may report incorrect values or set additional codes.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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