So your check engine light’s on and you have no freakin’ idea what’s wrong. Is something actually wrong with the engine, or is it just a failed sensor? If you’re driving an older vehicle — something that’s computerized and built before around 1995 — it’s probably OBD-I, which means two things: 1) You can’t use any of the fancy OBD-II code readers (with LCDs and built-in manuals) to tell what’s up, and 2) you aren’t going to have to spend $200+ on one of the aforementioned readers.
In fact, you can pick up a little “scanner” for arounde $25 that’ll do the job just fine, assuming that you’re patient enough to sit and count LED flashes.
By inserting the OBD-I “scanner” into the plug under the dash, you place the vehicle in “diagnostics mode.” When you then turn the key to the “on” position, the LED on the front of the device flashes out — in the case of GM vehicles — a series of two-digit codes. By referencing those codes in the accompanying manual, you can can figure out what the engine’s bitching about.
The code scanner pictured above is manufactured by Actron for use with GM vehicles, and it’s available for sale (under their name as well as a number of others like “Sunpro”) for around $25 online and at most auto parts stores. OBD-I implementation varies by manufacturer, but similar solutions are available for most other early computerized vehicles.