What causes Ford code 42?

A blocked intake or air filter restricts air and drives the system rich. Excessive fuel in the engine oil or high fuel pressure makes the mixture rich. A leaking injector overfuels the engine. A restricted breather or EVAP fault traps fuel vapor in the intake. The HO2S does not switch, so the ECM cannot correct the rich mixture.

How do I fix Ford code 42?

Check the intake and air filter for a blockage. Check for excessive fuel in the engine oil. Measure fuel pressure and compare it with spec, since this fault reads high. Check the injectors for a leaking one and check the fuel pressure regulator. Test the HO2S and replace it only if it fails its own test. Test the ECM last, only after every circuit test has passed, then clear the code and retest.

Ford 42 Fuel system indicates rich or oxygen sensor not switching

Q: What does Ford code 42 mean?

Ford code 42. Fuel system indicates rich or oxygen sensor not switching.

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

How to read it

Code 42 reads as 4 flashes, pause, 2 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 42 mean?

Fuel system indicates rich or oxygen sensor not switching

Common causes of 42

A blocked intake or air filter restricts air and drives the system rich.
Excessive fuel in the engine oil or high fuel pressure makes the mixture rich.
A leaking injector overfuels the engine.
A restricted breather or EVAP fault traps fuel vapor in the intake.
The HO2S does not switch, so the ECM cannot correct the rich mixture.

How to fix 42

Check the intake and air filter for a blockage.
Check for excessive fuel in the engine oil.
Measure fuel pressure and compare it with spec, since this fault reads high.
Check the injectors for a leaking one and check the fuel pressure regulator.
Test the HO2S and replace it only if it fails its own test.
Test the ECM last, only after every circuit test has passed, then clear the code and retest.

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. Code 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 EEC-IV trouble codes, Crown Victoria, Mustang, Taurus, Thunderbird 1992-1995 (code, fault location, 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 additional probable causes.
https://web.archive.org/web/20160502051413id_/http://www.troublecodes.net:80/ford/crown-victoria-mustang-cobralxgt-taurus-thunderbird-1992-1995/
Ford EEC-IV trouble code table: code, fault location, probable cause (Aerostar, Bronco, Econoline, Explorer, F-Series, Ranger 1992-1997)
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/aerostar-vanwagon-bronco-econolineclub-wagon-explorer-f-series-ranger-1992-1997/

Last updated 2026-10-10.

Report a correction for 42. Include your vehicle, the issue and a source if you have one.