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P1790

Transmission Control Module Checksum.

  • GMC
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
7
AreaTransmission
90
FaultThe specific fault in that area

What it means

P1790 on a GMC means the Transmission Control Module (TCM) has failed its internal checksum test. The module checks the integrity of its stored program and calibration data and found a mismatch, so it cannot trust its own memory. It may be a fault in the module itself, corrupted data, or a supply or programming problem.

Symptoms

  • Malfunction indicator lamp or transmission warning lamp on
  • Transmission stuck in one gear (limp mode)
  • Harsh, delayed or erratic shifts
  • No torque converter clutch lock-up
  • Poor fuel economy
  • Other transmission or communication codes stored alongside

Possible causes

  • Internal TCM memory or processor faultlikely
  • Corrupted or incomplete TCM software or calibration, for example after an interrupted updatepossible
  • Low or unstable battery or ignition supply to the TCM during programming or start-uppossible
  • Poor TCM power or earth connections, or corroded connector pinspossible
  • Water ingress or heat damage to the TCM (location and type vary by model; some are mounted in or on the transmission)possible
  • Outdated calibration that has a known checksum issue on certain models or enginesrare

How to diagnose

  1. Read all stored codes and freeze frame data, noting whether P1790 is current or history and whether other codes accompany it.
  2. Check battery condition and charging voltage, as low voltage can corrupt module memory.
  3. Inspect TCM wiring, connectors and earths for corrosion, damage, loose pins and water ingress. Where the TCM is located depends on model and transmission.
  4. Check the relevant power and earth feeds to the TCM against the wiring diagram for your model.
  5. Clear the code, then road test and rescan to see if it returns straight away or only intermittently.
  6. Check for any prior transmission repair or module programming that may have been interrupted.
  7. Look up manufacturer service information for calibration updates or bulletins that apply to your vehicle and transmission.

How to fix

  • Repair any faulty power, earth or connector problem found and clear the code.
  • Correct battery or charging system faults before any reprogramming.
  • Reprogram the TCM with the latest correct calibration where an update is available or the software is corrupted.
  • Replace the TCM if the fault persists after the checks, and program or set it up to the vehicle as the procedure requires.
  • Dry out and repair any water ingress source, and replace the affected module or connectors.

Can I keep driving?

Medium severity

The car may still drive but could be in limp mode with poor or harsh shifting. Get it checked soon, and avoid heavy loads or towing until the transmission is shifting normally.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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