1999 Ford Expedition: One coil not firing—PCM issue?

Hi all, I have a 1999 Ford Expedition, and I’m facing a frustrating issue where one ignition coil isn’t firing. Here’s what’s been done so far:

  • The faulty coil has been replaced twice, but the problem persists.
  • Two skilled mechanics have suggested the PCM might be at fault, but it seems unlikely since the other 7 coils are firing correctly.

Here are my questions:

  1. Could the PCM really cause an issue with just one coil, or is it more likely a wiring or connector issue?
  2. Is there a way to troubleshoot the wiring harness from the PCM to the coil before considering a PCM replacement?
  3. Has anyone encountered this issue before, and if so, how was it resolved?

Additional info:

  • I’ve been advised to inspect the grounds, ignition relays, crank and cam position sensors, and perform a bench test on the PCM.
  • I’m looking into replacing the wiring harness for the affected coil. Does anyone know how to identify the correct part number for this?

Any insights or suggestions would be greatly appreciated!

Yes, the PCM can cause a single coil to fail since each coil is controlled by individual pins on the PCM. If the pin for that specific coil is damaged, it won’t fire.

Zuri said:
Yes, the PCM can cause a single coil to fail since each coil is controlled by individual pins on the PCM. If the pin for that specific coil is damaged, it won’t fire.

Exactly. I had a similar issue, and it turned out to be a damaged PCM pin. Testing the output voltage at the PCM pin can confirm this.

Before replacing the PCM, check the wiring between the PCM and the coil. A loose or damaged wire can cause the same issue. A pinout diagram will help trace the connection.

Nicole said:
Before replacing the PCM, check the wiring between the PCM and the coil. A loose or damaged wire can cause the same issue. A pinout diagram will help trace the connection.

Here’s a link to an ignition coil wiring kit: Amazon link. It might be what you need if the wiring is the issue.

If you decide to replace the PCM, check for a refurbished or used one. New ones can be hard to find for a 1999 Expedition.

Oliver said:
If you decide to replace the PCM, check for a refurbished or used one. New ones can be hard to find for a 1999 Expedition.

Also, make sure the replacement PCM is properly programmed for your vehicle. Some vendors offer pre-programmed options.

Testing the wiring harness with a multimeter might save you a lot of guesswork. If the harness checks out fine, the PCM is likely the culprit.

I had the same problem with a misfiring coil. Turned out to be a loose ground near the PCM. Double-check all grounds before replacing expensive parts.