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Home / DTC / P1639 — 5 Volt Reference 2 Circuit

P1639 — 5 Volt Reference 2 Circuit

Detailed page for trouble code P1639.

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Code

P1639

CHEVROLET P — Powertrain

5 Volt Reference 2 Circuit

Brand: CHEVROLET
Type: P — Powertrain
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Page language: EN

Causes

  • Open or damaged 5V reference wiring or connector for Reference 2
  • Short to ground or short to battery voltage on the 5V reference circuit
  • Corroded or pushed-out connector pins at a sensor or the ECM
  • Failed sensor(s) on the Reference 2 circuit drawing excessive current
  • High resistance (poor crimp/repair) in the harness
  • Intermittent wiring fault from vibration or heat

Symptoms

  • Check Engine light (MIL) illuminated
  • Stored P1639 and possibly additional sensor-specific codes (MAF/IAT/TPS/etc.)
  • Intermittent or poor engine performance (stalling, hesitation) if affected sensors are critical
  • Unstable sensor readings on scan tool for sensors fed by 5V Reference 2
  • Vehicle may be able to run but with limp performance depending on which sensors are tied to Reference 2

What to check

  • Read codes and freeze frame with a scan tool; record related codes and PIDs for sensors on 5V Ref 2
  • Perform a clear visual inspection of harness, connectors, and grounds for damage or corrosion
  • Backprobe sensor connector(s) with key ON (engine OFF) and measure reference voltage with a DMM
  • Wiggle harness and connectors while monitoring voltage/scan data to find intermittent faults
  • Compare voltage at sensor connector to voltage at the ECM 5V Reference pin
  • Check for aftermarket additions or splices on the 5V reference circuit

Signal parameters

  • Expected 5 Volt Reference: approximately 5.0 V (typically 4.8–5.2 V) with key ON (engine OFF)
  • Reference circuit should be steady DC, not pulsing or switching
  • Voltage should be the same at the ECM pin and at each sensor feed (allow small drop over long runs)
  • Open-circuit or short to ground will read much lower (
  • Resistance: continuity between ECM reference pin and sensor connector should be low (typically

Diagnostic algorithm

  1. Record codes and live data. Note which sensors are on 5V Reference 2 using a wiring diagram or shop manual.
  2. Visually inspect connectors, harness routing, and any known trouble areas (engine mounts, firewall, near sharp edges).
  3. With key ON (engine OFF), backprobe the 5V Reference pin at one of the affected sensors and verify ~5.0 V. Repeat at each sensor on Reference 2.
  4. Measure voltage at the ECM 5V Reference 2 pin. If ECM pin is correct but sensors are low, inspect harness/continuity between ECM and sensors.
  5. Check continuity and for shorts: with battery disconnected, measure resistance from ECM reference pin to each sensor reference pin and to ground/12V to detect shorts or opens.
  6. If a single sensor shows abnormal readings, unplug that sensor and re-check the reference voltage at the harness connector under key ON. If voltage returns to normal, suspect that sensor is loading the circuit.
  7. Repair any damaged wiring, pins, or connectors. Replace failed sensor only after confirming it is the fault. Use proper crimping/soldering and insulation.
  8. If wiring and sensors test good and reference still abnormal at ECM, consider ECM internal failure — confirm with manufacturer-specific diagnostics before ECM replacement.
  9. Clear codes and perform a road test. Re-scan to verify the fault does not return.

Likely causes

  • Broken wire or pin-back at a sensor connector
  • Wire chafing to chassis ground or touching a 12V source
  • Contaminated or corroded connector causing high resistance
  • One sensor on the 5V Reference 2 circuit has failed and is loading the reference
  • Aftermarket accessory or previous repair created a poor splice or short

Fault status

⚠️ Status
DTC P1639 — 5 Volt Reference 2 Circuit: ECM detected abnormal voltage/condition on the Reference 2 5V supply (open/high/low/short/intermittent).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.5 hours

Similar codes

P1638 P0102 P0113 P0122 P0628
456

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