OBD-I Codes List

Ford 633 Four wheel drive switch closed during the KOEO test

Make
Ford (1981-1995)
Category
Powertrain - Computer and Auxiliary Controls
Severity
Check Engine Light

How to read it

Code 633 reads as 6 flashes, pause, 3 flashes, pause, 3 flashes on the check-engine light. Jumper the self-test input to signal return at the test connector, then run the key on engine off self test and the running self test, counting the check-engine flashes each time.

New to flash codes? The How to Read Ford EEC-IV Self-Test Codes walks through the test and what the numbers mean. The general OBD-I guide covers the rest.

What does Ford code 633 mean?

Four wheel drive switch closed during the KOEO test

Common causes of 633

  • The four wheel drive switch is closed when the KOEO test expects it open
  • A short to ground in the four wheel drive selector circuit
  • A failed four wheel drive selector switch
  • Internal damage to the transfer case keeping the switch closed

How to fix 633

  1. Check the four wheel drive selector position during the KOEO test
  2. Check the four wheel drive selector circuit wiring for a short
  3. Test the four wheel drive selector switch and replace it only if it fails its own test
  4. Check the transfer case for internal damage
  5. Test the PCM last, only after every circuit test has passed
  6. Clear the code, then retest to confirm the fix holds

Source

Source review recorded

Sources and review

Review: checked against the recorded source material.

  • EEC-IV Ford trouble code table (MIL, description, type)

    archived reference page (troublecodes.net via Wayback Machine)

    Supports: code identity and meaning

    Fetched directly. The three-digit row matches the description used in the data file.

    https://web.archive.org/web/20030720231430id_/http://www.troublecodes.net:80/Ford/eec-iv.shtml

  • Ford trouble codes, Contour, Escort, Mustang, Probe, Taurus and Tempo 1992-1995 (code, description, probable cause)

    archived reference page (troublecodes.net via Wayback Machine)

    Supports: code identity and probable causes

    Independent table on the same archive site with matching code identity and a probable cause column.

    http://web.archive.org/web/20160501223306id_/http://www.troublecodes.net:80/ford/contour-escort-mustang-probe-taurus-tempo-1992-1995/

  • Ford EEC-IV trouble code table: code, fault location, probable cause (Crown Victoria, Mustang, Taurus, Thunderbird 1992-1995)

    reference page (troublecodes.net)

    Supports: per-code probable causes used for the page cause list and test order

    Fetched 2026-09-30. The probable cause column is what the page lists, in source order, with the test order built from it.

    https://www.troublecodes.net/ford/crown-victoria-mustang-cobralxgt-taurus-thunderbird-1992-1995/

Last updated 2026-10-10.

Report a correction for 633. Include your vehicle, the issue and a source if you have one.

Related Powertrain - Computer and Auxiliary Controls Codes