Sunday, January 22, 2012

Musings on automotive diagnostics

Part of owning a car, and for me, part of having owned a LOT of cars, is dealing with a lot of problems. I've never had the money for a new car, and have thus never had a car in for warranty service. Why is this worth mentioning? Well, the long and the short of it is that any problem that crops up with the car is ultimately my responsibility.

Early on, this meant making a dreaded trip to the mechanic when some part or other would fail. Eventually, I started doing my own work, bit by bit. At this point, I haven't taken one of my own cars to a mechanic in about five years. Keep in mind I drive 25,000 miles a year or so, and have plenty that needs to be fixed on a regular basis.

Of course, before I can fix something, I need to determine what the problem is. Some things are obvious. Failed brakes spring to mind. As does worn-out suspension. But, when the car doesn't start, or doesn't run just right, I need to get into detective mode.

I had to do quite a bit of this last year, especially with the Vanagon and the 220D. The van had one problem after another until the engine failed on me in October. Even if I had been a bit quicker with my diagnostics, I wouldn't have been able to save it. The engine was just tired. As for the 220D, once I decided I needed to get it out of storage, I went through a lot of troubleshooting before the engine actually started for me.

With both of those cars, the thinking was all on me. I'd have to pay attention to what each car was (or wasn't doing) and use my best reasoning skills to figure out what the problem was. For the record, I did get the 220D to start and sold it running a little over a month ago.

Fast forward to this past week. The Legacy is my only running car at the moment, and takes me 45 miles each way in and out of work. If something breaks or otherwise malfunctions, I need to figure it out and fix it quickly. This past week, I was in exactly that situation.

On Tuesday, I was on my way home from work. It was wet and freezing, and I was on the highway. I was only a few miles into my journey when for no explicable reason, the "check engine" light, also known as a CEL or MIL (malfunction indicator light) came on. For me this was a new experience, as the 1995 Legacy I bought last year is the first car I've owned with a modern diagnostics system. This system, known officially as OBDII (on-board diagnostics 2) allows you to plug a scanner into the car and learn what the problem is.

I digress. I pulled off the road to check the car over. Nothing obvious was wrong. The car still seemed to have all its power and wasn't running any different. I opted to continue on my journey. When I got close to home, I stopped at the local Autozone. For those that don't know, Autozone will hook up a scanner and pull the trouble codes for you. This is convenient for me, as I don't own an OBDII scanner yet.

The clerk scanned the car and came up with a single trouble code - P0420 - Catalyst efficiency below threshold - bank 1. Long story short, this likely indicates a problem with an oxygen sensor or catalytic converter. I did some checking, and for my car the most likely candidate was the front oxygen sensor. Still, I decided to reset my engine computer and see if the code came back.

A little later in the week, I disconnected the car's battery while I was at work. As of today, several hundred miles later, the check engine light has not come back on again. Long story short, it looks like I got lucky, and the original "problem" was probably just a momentary sensor glitch, due to the cold, wet weather I was driving in on my way home.

So, the OBD codes aren't definitive, but they do help narrow down potential problems. As for me, I'm happy with the system. I'd much rather have a false alarm than have a sudden failure without any advance warning at all.



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