What causes Ford code 91?

The shift solenoid 1 circuit is open or damaged. Shift solenoid 1 itself failed. An intake leak drives the system lean and the HO2S reads lean. Low fuel pressure or a weak pump makes the mixture lean. The PCM cannot command shift solenoid 1.

How do I fix Ford code 91?

Check the wiring and connector at shift solenoid 1 in the transmission. Test shift solenoid 1 and replace it only if it fails its own test. Check the intake for a leak and the HO2S for a lean signal. Measure fuel pressure and compare it with spec, since the lean half of this code reads low. Test the PCM last, only after every circuit test has passed. Clear the code, then retest to confirm the shift and the mixture are correct.

Ford 91 Shift solenoid 1 circuit failure or oxygen sensor indicates lean

Q: What does Ford code 91 mean?

Ford code 91. Shift solenoid 1 circuit failure or oxygen sensor indicates lean.

Make: Ford (1981-1995)
Category: Powertrain - Transmission
Severity: Check Engine Light

How to read it

Code 91 reads as 9 flashes, pause, 1 flash 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 91 mean?

Shift solenoid 1 circuit failure or oxygen sensor indicates lean

Common causes of 91

The shift solenoid 1 circuit is open or damaged.
Shift solenoid 1 itself failed.
An intake leak drives the system lean and the HO2S reads lean.
Low fuel pressure or a weak pump makes the mixture lean.
The PCM cannot command shift solenoid 1.

How to fix 91

Check the wiring and connector at shift solenoid 1 in the transmission.
Test shift solenoid 1 and replace it only if it fails its own test.
Check the intake for a leak and the HO2S for a lean signal.
Measure fuel pressure and compare it with spec, since the lean half of this code reads low.
Test the PCM last, only after every circuit test has passed.
Clear the code, then retest to confirm the shift and the mixture are correct.

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