Auto PartsTech Tips

Check engine light diagnostics

The primary reason I end up doing so much work on my own car is that I drive older cars. I’ve heard stories and songs telling of warranty work on cars. A problem pops up and the owner turns the car over to the dealership, who in turn, fixes it. This, unfortunately, has never been the experience for me. Every time a new problem pops up, I need to fix it. Sometimes I need to figure it out first. Such has been the case for me recently.

This past weekend I was out for a drive and the check engine light came on. I’ve had this car for a little over a year now. It’s my first car with an OBDII system, and being a 1995 Subaru, it was their first attempt at integrating it into one of their cars. OBDII became standard for all cars sold in the US in 1996. OBDII stands for On Board Diagnostics. The “II” indicates that it’s the second generation of electronic engine monitoring.

As soon as I could, I got the car scanned. As the car is OBDII, there is a scan port integrated into the dashboard which allows you to hook up a code reader and get a readout of the problem. In my case, the only code I had was P0302, which indicates a misfire on cylinder 2. It’s worth noting that these codes, also called DTCs, or Diagnostic Trouble Codes, rarely spell out exactly which component or components have failed. Rather, these codes indicate specific defects in vehicles. It’s up to the mechanic to figure out the cause of the incorrect operating condition.

This, as usual, is where my brain comes in. Engines, at their most basic, need three things to run – air, fuel, and spark. An air supply problem would likely affect all cylinders, so I’m discounting that as a possibility for the moment. The car was running and had a half tank of gas, so I can effectively rule out a fuel supply problem. That leaves the fuel injection system and ignition systems as the most likely causes of the DTC.

As I do when I approach all automotive problems, I start by checking simple items and then move on from there. Spark plugs are cheap, so I cleared the code and threw in a new set to see if that cured the misfire. The code came back a few miles down the road. Knowing that the misfire is either an ignition or fuel issue at that cylinder, and that I have brand new spark plugs, I’m going to be replacing the coil pack and plug wires with known good ones. If the problem persists, I’ll do a compression test. Pending the results of that, I may end up replacing t

he fuel injector.

Bottom line, an OBDII system will describe the problem your engine is having, but won’t tell you the cause. It’s up to you to figure it out from there.

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