Scott,

The "dummy lights", such as the check engine light, for the early model Eclipses are extremely vague when it comes to diagnosing a problem. But I may have some things for you to check.

Your friend may be correct about the oxygen sensor. Depending on the style of Eclipse you have (SOHC, DOHC, turbo) will determine how difficult that sensor will be to check/replace. You can tell if your oxygen sensor is failing because your exhaust will smell like it's running rich and your gas mileage will be very bad. There will be a lot of black soot on the tip of the muffler. Some places, such as major chain auto parts stores, can check your oxygen sensor for you if you bring it in to them. Many of them will even loan you a special tool for free that'll help remove your O2 sensor.

The problem could also be coming from your air box; that big, black can on the passenger side, near the front of the engine. Your air filter could be so dirty that you're not getting good air flow. Or, you may have some blockage in one of your intake hoses. There is a 6 or 8 pin connector on your air box (again, depending on the model you have) that sends readings to the car's computer (ECU) that measures air temperature and barometric pressure. If that Mass Air Sensor (MAS) is malfunctioning it can cause your check engine light to come on.

Now, those can all have generally easy fixes, and can be detected easy enough with procedures from a repair manual made for your car; Haynes is a good one.

But there may be a more notorious problem. The 1G Eclipses (first generation, '90-'94) that Mitsubishi produced came with ECUs with bad capacitors in them. Over time, these capacitors would leak their fluid onto the circuit board of the car's computer, causing sensors on the computer to short out. If this is the case, you'll need a new computer for your car. A new one can cost up to $1100 from the dealership... but you can find a rebuilt one for around $100 when you send in your shorted one.

If you find out that your ECU (car's computer) is fine, you should still have those capacitor's changed. I have lots of experiences with circuit boards, so I was able to perform the task myself; and it only cost me about $2. There are many sites on the internet that know of this problem and can walk you through the fix; and will also sell you the correct capacitors for a couple bucks. If you try it yourself, and screw it up, you are screwed. But you can buy the capacitors and take your ECU to a local TV repair place and they will probably perform the task for you.

These are just some possible causes for your check engine light to come on. The ones mentioned above are the most probable. You could do all these repairs yourself, but if you have any doubt take the car to some one that knows what their doing.

Good Luck!