90s Subaru Legacy with an electrical mystery

Hello all,

I’ve got a 99 Subaru Legacy (30th anniversary edition!) sedan with the 2.2L engine and a handful shy of 100k miles. She’s been struggling with some sort of electrical bug. I’ve mysteriously burned out 4 coil packs in the last 9 months, or about one every 3000 miles. 3 of the times, one or two plugs would stop firing first, leaving the car drivable but shaky.

The other time, the engine cut while I was driving and wouldn’t turn back on. Turned out I’d blown a fuse responsible for the ECM, but every replacement fuse blew until the coil itself was replaced. This leads me to envision something was running hot inside the coil pack, melted thru insulation, and shorted out the circuit leading to that fuse. Since the coil packs keep going, I suspect the electrical issue lies here.

I’ve had a P0420 code (i.e. low cat efficiency) forever, and I’ve heard this is usually caused by the O2 sensor going bad. No nasty smells, so I’m praying my cat is still good. Periodically, I get slight headlight dimming during braking. Also, right around the time the coils started burning out, my left blinker started blinking at erratic and unpredictable rates. The front and rear lights turn on at opposite times rather than in sync.

Mechanics wanted to replace my whole wire harness, but I’m not trying to blow $6,000 yet if a coil pack is only $80 and i can replace it in 5 minutes on the side of a highway. Any ideas for how to tackle this? If it helps, I’m an engineering student armed with a $30 multimeter and a soldering iron. Thanks for anything you can offer!

Check that all ground connections are clean and tight.

Pull your spark plug(s) and verify they are the correct plugs. Using plugs with too high resistance can damage coils.

I’ve also heard that a lean condition caused by fuel injectors delivering too little gas can cause the spark plugs to run too hot, increasing their resistance, and thus damaging coils. Perhaps your P0420 code has something to do with it.

Also, if you have a slight head gasket leak that is allowing some coolant into one or more cylinders, that would cause your P0420 code and also result in lean/misfire conditions that would eventually burn out a coil.

Given the notorious had gasket problems of Subies I’d go out on a limb and suspect the head gasket. Find out why you have a P0420 code (bad head gasket?) and I bet that will also explain the burned out coils.