OBD-I Codes List

Ford 139 No HO2S switch detected

Make
Ford (1981-1995)
Category
Powertrain - Fuel/Air Metering
Severity
Check Engine Light

How to read it

Code 139 reads as 1 flash, pause, 3 flashes, pause, 9 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 139 mean?

No HO2S switch detected

Common causes of 139

  • The HO2S never switched, so the ECM stored no switching activity.
  • The HO2S wiring or connector is open or damaged.
  • The HO2S itself failed and stays flat.
  • An intake leak or a clogged injector keeps the mixture from crossing the switch point.

How to fix 139

  1. Check the HO2S wiring and connector for an open or damage.
  2. Test the HO2S and replace it only if it fails its own test.
  3. Check for an intake leak or a clogged injector that keeps the mixture flat.
  4. Test the ECM last, only after every circuit test has passed.
  5. Clear the code, then retest to confirm the HO2S switches.

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 139. Include your vehicle, the issue and a source if you have one.

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