What causes Ford code 18?

The spark output circuit between the ICM and the PCM is open. The ignition diagnostic signal circuit is damaged or disconnected. The ICM failed and no longer sends the spark output. The PCM cannot read the ignition diagnostic signal.

How do I fix Ford code 18?

Check the spark output circuit and the ignition diagnostic signal circuit for an open. Check the connector pins at the ICM and the PCM for corrosion, spread terminals or a poor fit. Test the ICM and replace it only if it fails its own test. Test the ECM last, only after every circuit test has passed. Clear the code, then retest to confirm the spark output signal returns.

Ford 18 Spark output circuit open or ignition diagnostic signal fault

Q: What does Ford code 18 mean?

Ford code 18. Spark output circuit open or ignition diagnostic signal fault.

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

How to read it

Code 18 reads as 1 flash, pause, 8 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 18 mean?

Spark output circuit open or ignition diagnostic signal fault

Common causes of 18

The spark output circuit between the ICM and the PCM is open.
The ignition diagnostic signal circuit is damaged or disconnected.
The ICM failed and no longer sends the spark output.
The PCM cannot read the ignition diagnostic signal.

How to fix 18

Check the spark output circuit and the ignition diagnostic signal circuit for an open.
Check the connector pins at the ICM and the PCM for corrosion, spread terminals or a poor fit.
Test the ICM and replace it only if it fails its own test.
Test the ECM last, only after every circuit test has passed.
Clear the code, then retest to confirm the spark output signal returns.

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