P1621
Control Module Long Term Memory Performance Condition
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The P1621 code is a GM-specific code indicating that the PCM's long-term memory (EEPROM) has failed a self-check. The EEPROM stores critical preprogrammed data that the PCM needs to run the engine and transmission — data that should persist even when the battery is disconnected. When the checksum of this data doesn't match expected values, the PCM sets this code.
This can be caused by something as simple as a low battery voltage during a critical moment, or it can indicate that the PCM itself is failing. Before assuming the worst, check the battery and all power and ground connections to the PCM. A corroded ground strap is a common and cheap fix. If the connections are solid and the battery tests good, the next step is to try reprogramming (reflashing) the PCM at a dealer or shop with GM diagnostic equipment.
If the PCM cannot be reprogrammed successfully, it will need to be replaced. A new or remanufactured PCM for a GM vehicle typically costs $300–$800, and it must be programmed with the vehicle's VIN, options, and calibration data after installation. This is not a typical DIY repair due to the programming requirement. The vehicle may be drivable with this code, but behavior can be unpredictable, so it's best to address it promptly.
Severity
Symptoms
- •Check engine light is on
- •Engine may stall or run erratically
- •Transmission may shift harshly or at wrong times
- •Vehicle may not start or may crank but not fire
- •Multiple other warning lights may illuminate
Likely Causes
The PCM's internal Electronically Erasable Programmable Read Only Memory (EEPROM) has become corrupted, failing checksum verification. This can happen due to voltage spikes, age, or manufacturing defects.
A weak battery or charging system issue can cause voltage drops during PCM operation, corrupting stored data in the EEPROM. This is especially common after jump-starts or battery replacements.
The PCM itself has an internal hardware failure in its memory circuits, preventing reliable data storage and retrieval.
Corroded or loose ground straps and power supply connections to the PCM can cause intermittent voltage drops that corrupt memory data.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Check and secure all PCM ground straps and power connections
- Test battery and charging system — replace weak battery
- Attempt PCM reprogramming (reflash) at a dealer or qualified shop
- Replace the PCM if reprogramming fails
- Check for and address any voltage supply issues to the PCM