Been driving my 2012 Ford Focus for over 120,000 miles, and I thought I’d seen it all. A few weeks back, I started noticing a rough idle and occasional misfire—especially under acceleration—and the check engine light popped on with a misfire code (P0302, indicating cylinder 2). The local dealership jumped straight to the usual culprit: a faulty ignition coil. They claimed it was an expensive, time-consuming fix. As a longtime skeptical owner who’s seen more than his fair share of overblown “symptoms,” I decided to do a bit of my own digging before handing over another wad of cash.

I pulled out my OBD-II scanner, confirmed the misfire on cylinder 2, and then started inspecting the spark plugs and coil connectors myself. I replaced the spark plugs thinking they might’ve worn out (they were nearly 25,000 miles old), but the misfire persisted. I wasn’t convinced that the coil itself was the issue either; after all, the replacement cost was steep and this wasn’t the first time I’d seen a misdiagnosis that’d led to unnecessary expenditure.

So, I took a closer look at the wiring harness leading to the ignition coil on cylinder 2. With my digital multimeter, I noticed an inconsistent voltage reading compared to the other cylinders. Carefully disconnecting the connector, I saw a hint of corrosion on one of the contact pins. It turns out that moisture had gradually built up inside the poorly sealed connector, causing intermittent poor contact and the misfire code.

The fix was straightforward—cleaned the connector thoroughly with a high-quality electrical contact cleaner, re-seated it, and secured it with dielectric grease to help prevent future corrosion. After a quick test drive, the rough idle was gone, the misfire disappeared, and I saved myself the expense of a brand-new ignition coil.

Lessons learned:

Don’t always trust the first diagnosis, especially if it comes with a hefty bill.
Trust your tools—using an OBD-II scanner and multimeter can often reveal issues that aren’t as catastrophic as they first seem.
Sometimes the smallest, simplest issues like a corroded connector can mimic symptoms of major component failures.

If you experience a misfire or rough running engine, take a moment to inspect those wiring connections before jumping into component replacements. It might just save you time and a lot of money.