I have owned a Volvo V70-T5 for the last 13 years. After 136,000(+) miles, I have come to the conclusion that having the local Volvo dealer do all the service is not always the best option. Recently I replaced the two exhaust mufflers and connecting pipe at a cost savings of $900(+) below the dealer's cost estimate. Therefore, when the "pesky" Check Engine light comes on, I am wondering what type of problem(s) may be causing this. I noticed that some fluid was in the exit pipe of the catalytic converter when I replaced the mufflers. The light Check Engine light has come on and gone off by itself before. I had though that replacing the exhaust system might solve the problem. The Owner's Manual states that this is likely a "emission problem" and "see your dealer" is recommended. the car runs fine and uses no oil.
My Question: What are the possible problems that may cause this.