What does P1791 mean on a Chevy?

P1791. Transmission Control Module Loop. Applies to Silverado, Sierra, Tahoe, Yukon, Suburban.

What causes P1791 on a Chevy?

Faulty transmission control module failing its internal loop check Open or shorted TCM harness from heat and vibration near the transmission Poor TCM ground raising reference voltage inside the module Low system voltage during crank corrupting the TCM self-test

How do I fix P1791 on a Chevy?

Check battery and charging system health before condemning the TCM Inspect TCM powers, grounds, and connectors for corrosion and tension Test the TCM harness for opens and shorts while wiggle-testing Clear the code and rerun the TCM self-test with a Tech2 or GDS2 Replace the TCM only if the loop fault returns with good powers and grounds

Code

P1791

Transmission Control Module Loop

Category: Powertrain - Transmission
Severity: Check Engine Light
Applies to: Silverado, Sierra, Tahoe, Yukon, Suburban
Related: No single generic twin. See OBD Codes List

What it means

P1791. Transmission Control Module Loop. This Chevy-enhanced code applies to Silverado, Sierra, Tahoe, Yukon, Suburban.

Findings, check these first

Faulty transmission control module failing its internal loop check
Open or shorted TCM harness from heat and vibration near the transmission
Poor TCM ground raising reference voltage inside the module
Low system voltage during crank corrupting the TCM self-test

Standard procedure

Check battery and charging system health before condemning the TCM
Inspect TCM powers, grounds, and connectors for corrosion and tension
Test the TCM harness for opens and shorts while wiggle-testing
Clear the code and rerun the TCM self-test with a Tech2 or GDS2
Replace the TCM only if the loop fault returns with good powers and grounds

Look up another code

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Sources and review

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Last updated 2026-09-19.

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