Hello-
What model Volvo?
And how does the vehicle run? What code was it? I'm assuming XXXXXX?
Well, on the old body style S40's, the ECM is not capable of determining which cylinder is misfiring, though it has been reported that it will throw misfire codes for specific cylinders when using generic P-Codes. The Volvo code that is set is ECM-44 which just denotes a misfire and can be a hassle to track down. I see you have replaced the spark plugs already, but there are a number of other things that can cause this code. Check for leaking or blocked fuel injectors. Also, remove the vacuum line to the fuel pressure regulator to check for leakage. If fuel comes out, replace the regulator. Also check for moisture in the spark plug wells as well as examining the plug wires. It could be an ignition coil but you can't test them because the computer can't tell what cylinder is misfiring (Normally you would swap coils with another cylinder and see if the code moves to that cylinder). Perform a compression test and if necessary, a cylinder leakage test. Make sure the oil is at the proper level and not saturated with fuel. If you have access to a smoke machine, check the intake system for leakage.
Volvo Master Technician
ASE Master & L1 Certified, Volvo Master Technician, EPA 609 Certified