What causes the check engine light to go on for a Volvo 850 Turbo?

I have been working with an 850 GLT 2.4 Turbo for the last year. This vehicle had an EVAP problem when I bought it. AND, I ended up doing a head job on in it. After my daughter let it get to hot. You first need to buy or rent a OBD-II scanner tool. these can be rented at a local parts store. Run the code test and it will give you a code.


My experiences have been the EVAP and Coolant problems.
If it is the EVAP issue then you need to look at the fuel tank to see if there is a leak present. There is a recall on the fuel tank if it is leaking on the upper side next to the tail pipe. There is also a a few lines that could be leaking in the fuel and secondary air system. Just a simple crack can cause this. In this case don't waste any money take it to a certified repair shop and have a smoke test accomplished. Then it will be pin pointed real quick and easy. This could run from $75-$150. DEPENDING.
Codes encountered- P0442


Coolant issue: There are many reasons here. Leaking head gasket, bad temperature sensor, temp relay, coolant fan, etc. You can do a few simple tests. The head gasket is a little involved. A scale on working skill 1-5. I would put it at a level 3 or 4 depending on your knowledge on how to work on cars. If the water container is low. Fill as recommended by manufacturer. Drive around and look to see if quantity has gone down. If it has you can purchase a coolant test kit from Harbor Freight accomplish test from Haynes manual or ALLDATADIY.COM. Removing all spark plugs and pressurizing the system, You will see or here water bypassing into cylinders or outside the head. The engine will not use water at idle sitting still you have to drive the vehicle. Very expensive if it needs shop work. Bad temp sensor: My encounter was engine would start cold and hard start while warm or hot. This could be a wire problem. Fan Relay you have to listen when fan runs at idle and after driving. read Haynes manual for explanation. There is a sensor in the tank. Always make sure this existent. This item could cause you great heart ache.
Codes encounter- P0117


Recommendations: If you are going to keep this vehicle for a while I recommend the ALLDATADIY.COM site. Very inexpensive and worth it as a guideline. This is a shop repair manual and some test you have to compensate for. Also the Haynes Manual. Also recommend if you replace the head gasket use only Volvo parts.
Reminder: Parts on this vehicle are very expensive.