OBD-I Codes List

GM 66 A/C Pressure Sensor Circuit Low Pressure Indicated

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

How to read it

Code 66 reads as 6 flashes, pause, 6 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 66 mean?

A/C Pressure Sensor Circuit Low Pressure Indicated

Common causes of 66

  • Low refrigerant charge
  • Failed A/C pressure sensor
  • Open sensor signal wire
  • Faulty sensor ground

How to fix 66

  1. Read codes with an ALDL scan tool
  2. Compare sensor reading with manifold gauge pressure
  3. Inspect the sensor connector and wiring
  4. Check system charge before condemning the sensor
  5. Replace the sensor if gauges read normal but signal reads low
  6. Evacuate and recharge only after the leak is fixed

Source

Qualified source review

Sources and review

Review: qualified. Read the source limits below.

  • GM and Chevrolet OBD1 Trouble Codes (ALDL Fault Code List)

    live reference page (troublecodes.net)

    Supports: code identity and GM meaning

    Fetched directly. Single publisher, so the record is qualified, not verified. Causes and check order are curator-written from the circuit the code names.

    https://www.troublecodes.net/gm/

Last updated 2026-09-19.

Report a correction for 66. Include your vehicle, the issue and a source if you have one.

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