buy or rent an OBD2 scanner and find the fault codes that set off the CEL is the first step. There's some new ones with bluetooth that can let you read data into a laptop or phone app. I have a $39 scanner from Harbor Freight that has saved me some $$ over the past couple years

 

 

 

Now, a year later - what ?
I also have "REP" - reduced engine performance . and its SERIOUS ! and I am becoming dispirit...
I have one of those HF scanners, but this one does not work with the 2003 Volvo ..
This engine, in the '03 S60, T,5,AWD, 140K miles, has been pressure washed.. Now I worry .

 

With any luck, the electronic module that's most likely causing the problem will dry out and everything will be fine again. Or not - but most times it does work out that way.

 

Your 2003 Volvo is compatible with the standard OBD2 protocols. Like I said, you can get a $30 reader from Harbor Freight that'll work (used it on my 2000 S40, 2004 Highlander and 2012 VW CC with no issues). +1 with Habby about waiting for the engine to dry... Not sure why you felt the need to pressure wash a place where there's lots of wires and small bits that can be blown off...

 

Thanks for the replies
I have the $29-30 HF scanner, but now we are having Ridgefield, Conn. weather , maybe worse .
During the summer, the engine and its bay were carefully pressure washed - particularly in the cooling reservoir area. ..Many say ''do not pressure wash the engine" . Now I believe them .