μblog: engineering from the trenches

For the first time in almost 170,000 miles, the Check Engine light on my 99 Jeep Grand Cherokee came on. I try to take really good care of the vehicle so it has only had minor problems so far. Fearing for the worst, I did a few web searches and confirmed that this model Jeep (and almost all of the later models) have a feature where the trouble code can be viewed on the digital odometer. By turning the ignition key on-off-on-off-on in a short period of time (~5s) the Jeep is persuaded to display the error code(s) in the odometer window followed by a “done”. After checking the table in the Hayes auto repair manual, I confirmed that it was the throttle position sensor that was malfunctioning. Although the sensor was in an odd place, at the back of the throttle body, the repair was pretty easy and relatively quick. The potential lesson here is that if your check engine light comes on, there may be a simple way to retrieve the diagnostic code yourself leading you to either repair the problem yourself, if it is simple, or knowing what type of problem it is before taking it to a mechanic.