OBD-I Codes List

GM 99 Invalid PCM Program

Make
GM (1982-1995)
Category
Powertrain - Computer and Auxiliary Controls
Severity
Check Engine Light

How to read it

Code 99 reads as 9 flashes, pause, 9 flashes on the check-engine light. Jump terminals A and B on the ALDL connector with the key off, then turn the key on with the engine off and count the check-engine flashes.

New to flash codes? The How to Read GM OBD-I Codes with a Paperclip walks through the test and what the numbers mean. The general OBD-I guide covers the rest.

What does GM code 99 mean?

Invalid PCM Program

Common causes of 99

  • Incorrect MEMCAL or flash calibration installed in the PCM
  • Poor contact at the MEMCAL socket pins in the PCM
  • Internal PCM memory fault corrupting the stored program
  • Incorrect programming after a PCM replacement

How to fix 99

  1. Pull codes at the ALDL port and record every stored fault first
  2. Disconnect battery power, remove the PCM cover and check MEMCAL seating
  3. Check the calibration part number against the factory VIN listing
  4. Look for green corrosion on the PCM connector pins and MEMCAL socket
  5. Install a known good MEMCAL or reflash the PCM for the correct drivetrain
  6. Swap the PCM only if the program still will not validate, then clear the code and retest

Source

Qualified source review

Sources and review

Review: qualified. Read the source limits below.

  • GM OBD1 trouble codes, model and year page

    live reference page (troublecodes.net)

    Supports: code identity and GM meaning

    Fetched directly from the linked model page. Single publisher, so the record is qualified, not verified.

    https://www.troublecodes.net/GM/90_453/

Last updated 2026-10-10.

Report a correction for 99. Include your vehicle, the issue and a source if you have one.

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