What does P1639 mean on a Chevy?

P1639. 5 Volt Reference 2 Circuit. Applies to Silverado, Sierra, Tahoe, Yukon, Corvette.

What causes P1639 on a Chevy?

Shorted sensor on reference 2 bus (commonly APP sensor, TP sensor, or EGR pintle) Chafed reference 2 harness shorting to ground Failed PCM reference 2 regulator Oil or coolant intrusion in the accelerator pedal position connector

How do I fix P1639 on a Chevy?

Identify which sensors share reference 2 for the model year, then unplug them one at a time Monitor reference 2 voltage on a Tech2/GDS2 scan tool while disconnecting sensors Inspect the APP sensor connector above the pedal for liquid intrusion Replace the failed sensor; replace the PCM only if reference 2 stays low with everything unplugged

Code

P1639

5 Volt Reference 2 Circuit

Category: Powertrain - Computer and Auxiliary Controls
Severity: Check Engine Light
Applies to: Silverado, Sierra, Tahoe, Yukon, Corvette
Related: No single generic twin. See OBD Codes List

What it means

P1639. 5 Volt Reference 2 Circuit. This Chevy-enhanced code applies to Silverado, Sierra, Tahoe, Yukon, Corvette.

Findings, check these first

Shorted sensor on reference 2 bus (commonly APP sensor, TP sensor, or EGR pintle)
Chafed reference 2 harness shorting to ground
Failed PCM reference 2 regulator
Oil or coolant intrusion in the accelerator pedal position connector

Standard procedure

Identify which sensors share reference 2 for the model year, then unplug them one at a time
Monitor reference 2 voltage on a Tech2/GDS2 scan tool while disconnecting sensors
Inspect the APP sensor connector above the pedal for liquid intrusion
Replace the failed sensor; replace the PCM only if reference 2 stays low with everything unplugged

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Sources and review

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Last updated 2026-09-13.

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