Corrupted or failed TCM internal flash memory, which is the most common root cause on high-mileage GM units.; Low or unstable battery voltage during cranking or while the TCM is writing adaptive data, which can corrupt the checksum.; Poor ground connections at the TCM or at the transmission case, causing voltage spikes that damage stored data.; Water intrusion into the TCM connector or case, leading to corrosion and intermittent memory faults.; A failed or failing ignition switch that drops power to the TCM mid-write cycle.; Aftermarket tuning or a botched reflash that left the calibration checksum incorrect.; Rarely, a wiring harness chafe that shorts the TCM power or ground circuit and disrupts memory integrity.

DTC codes / GM codes / P1790

P1790 · GMTransmission Control Module Checksum

Q: What does code P1790 mean?

Transmission Control Module Checksum. P1790 on a GM vehicle means the Transmission Control Module (TCM) has detected a checksum error in its own internal memory or in the calibration data it relies on to control shifting. The TCM runs a mathematical check on stored data every time the ignition is turned on, and when the calculated value does not match the expected value, it flags this code and typically commands a default or limp-home shift strategy.

P1790 on a GM vehicle means the Transmission Control Module (TCM) has detected a checksum error in its own internal memory or in the calibration data it relies on to control shifting. The TCM runs a mathematical check on stored data every time the ignition is turned on, and when the calculated value does not match the expected value, it flags this code and typically commands a default or limp-home shift strategy.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyProfessional

AreaTransmission

Symptoms you may notice

The Service Engine Soon or Transmission warning light illuminates on the instrument cluster.
Often no symptoms besides the warning light, especially if the checksum fault is intermittent.
Harsh or delayed shifts as the TCM falls back to a default pressure and shift schedule.
The transmission may stay in a single gear or refuse to downshift when coming to a stop.
In some cases the vehicle enters limp mode with reduced engine power or limited road speed.
A no-start condition can occur if the TCM is part of the vehicle's anti-theft or communication network and the checksum failure prevents proper handshaking.

Common causes, most likely first

Corrupted or failed TCM internal flash memory, which is the most common root cause on high-mileage GM units.
Low or unstable battery voltage during cranking or while the TCM is writing adaptive data, which can corrupt the checksum.
Poor ground connections at the TCM or at the transmission case, causing voltage spikes that damage stored data.
Water intrusion into the TCM connector or case, leading to corrosion and intermittent memory faults.
A failed or failing ignition switch that drops power to the TCM mid-write cycle.
Aftermarket tuning or a botched reflash that left the calibration checksum incorrect.
Rarely, a wiring harness chafe that shorts the TCM power or ground circuit and disrupts memory integrity.

How to diagnose and fix it

Connect a scan tool and record all TCM codes plus freeze-frame data before clearing anything, because the checksum code may be a one-time event or may return immediately.
Check battery voltage and load-test the battery; a weak battery that drops below 9.5 volts during cranking is a frequent trigger for memory corruption.
Inspect and clean the TCM ground points at the transmission and chassis, and verify less than 0.1 volt drop on the ground circuit under load.
Disconnect the TCM connector and inspect for water intrusion, corrosion, or pushed-back pins; dry and repair as needed.
Clear the code and perform a relearn or adaptive reset with the scan tool, then drive the vehicle through a full shift cycle to see if the code returns.
If the code returns immediately, reflash the TCM with the latest GM calibration using a J2534 pass-through device or a factory scan tool.
If reflashing does not resolve it, replace the TCM and perform the required programming and anti-theft relearn for the vehicle.

What a checksum code really tells you

A checksum error does not always mean the TCM is dead; a single low-voltage event during cranking can set P1790 once and never return after clearing.
Many technicians replace the TCM only to find the real culprit is a corroded ground strap on the transmission, so always verify ground integrity before condemning the module.
Aftermarket handheld tuners and improper reflashes are a common cause on GM trucks and SUVs, and the fix is often a correct factory calibration rather than new hardware.
P1790 can be stored alongside communication codes like U0100 or U0101 when the TCM drops offline, which can mislead you into chasing a CAN bus fault instead of the memory fault.
On some GM platforms the TCM is inside the valve body and shares fluid with the transmission, so overheating or contaminated fluid can precede internal electronics failure.

Can I keep driving with P1790?

You can usually drive the vehicle to a shop if it is shifting normally, but avoid heavy throttle and highway speeds because the TCM may suddenly default to a limp-home gear. If the transmission is already harsh-shifting or stuck in one gear, limit driving to a short distance to a repair facility.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.