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P1790

Transmission Control Module Checksum.

  • Buick
  • 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

On a Buick, P1790 means the transmission control module (TCM) failed its internal checksum test. The module checks its stored program and calibration data against an expected value, and the result did not match. This points to corrupted memory or a faulty module rather than a sensor or wiring fault, though a poor power supply can sometimes trigger it.

Symptoms

  • Malfunction indicator lamp or transmission warning lamp on
  • Harsh, delayed or flared gear changes
  • Transmission stuck in one gear (limp mode)
  • No shift into higher gears or loss of torque converter lock-up
  • Fixed or default shift pressures
  • Fault returns soon after being cleared

Possible causes

  • Internal fault or corrupted memory in the transmission control modulelikely
  • Outdated, corrupt or incomplete calibration in the TCM, for example after an interrupted software updatepossible
  • Low or unstable battery voltage, or poor power and earth supply to the TCMpossible
  • Water ingress or heat damage inside the module or its connectorpossible
  • Voltage spikes from a faulty charging system or poor jump-startrare
  • Wiring or connector damage causing intermittent power loss to the TCMrare

How to diagnose

  1. Read all stored codes and freeze frame data, noting whether other TCM or network codes are present alongside P1790.
  2. Check battery condition and charging voltage, as low or unstable voltage can corrupt module memory.
  3. Inspect the TCM connector, power feeds and earths for corrosion, looseness, water or damage.
  4. Check the TCM housing and mounting location for moisture, heat damage or physical damage. Its position depends on the model and transmission.
  5. Clear the code and road test to see whether it returns immediately or only intermittently.
  6. Check whether there is a calibration update or service bulletin for the vehicle's TCM and transmission type.
  7. Check module power and earth voltage drop under load before deciding the TCM is faulty.

How to fix

  • Repair any low battery or charging system fault, then clear the code and retest.
  • Repair or replace damaged TCM power supply wiring, earths or connectors.
  • Reprogram the TCM with the latest correct calibration for the vehicle if an outdated or corrupt calibration is confirmed.
  • Replace the TCM if the fault persists after power supply checks and reprogramming, then program and set it up to suit the vehicle.
  • Dry and repair the area affected by water ingress and replace any damaged module.

Can I keep driving?

Medium severity

The car can usually be driven short distances, but the gearbox may default to a fixed gear or shift harshly, which is unpredictable and can stress the transmission. Get it diagnosed soon and avoid long journeys or towing.

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

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