Ok, Error code is for mis-fire #4 cylinder. Checked spark plugs, wires, coils. did a back pressure test for a P0420 code that showed up at the same time. Everything looks good.
Detail performance issues: Car starts fine, runs pretty smooth in park or nuetral, but when you drive, forward or reverse, i.e. put a load on the engine, it runs very sluggish and slow to pick up speed. the harder you push on the gas, the more the check engine light blinks.
Now for the oddity: there is a plug to the left forward of cylinder #4 that I unplugged, this started the radiator fans, car ran great. Fans didn't turn off till I reconnected the plug and switched the key to on. I can't tell where the wires to this plug are going and thought somone might have experienced this. It has to be some type of sencor or something in my thinking. Thanks for any help

 

Did you unplug the coolant temp sensor? Are the wires leading to the thermostat housing? Other than the CVVT solenoid I'm not picturing anything in that area.

And I'd bet you need new ignition coils. Mine ohm'ed fine but still the car ran poor until I replaced both coils. When is the last time you replaced the ignition coils?

 

I second on the coils but whatever did you unplug you need to plug it back.The 4th cylinder is on the right side of the engine but there is nothing to unplug other than the coil itself.
Would be also a good idea to check for vacum leaks.

 

you unplugged the maf sensor

 

GUYS,
I need help I am very new to the forum. I have 2001 volvo s40 that has engine light on for p304 cylinder 4 misfire. This is what I have done so far to get it done.
Changed spark plugs
changed coil and cables
got cleaned fuel injectors.
After doing all this engine light did not go away even I did reset it for severl time.
now after fuel injector cleaning vehicle is consuming too much gas.
but vehicle runs very smooth sounds like have no issue but damn engine light has become a headache.
Please help me

 

try swapping the injector with another cylinder. Sounds like it could be leaking or it's constantly energized.

 

Check the spark plugs color too. A cylinder running too rich will cause a black, soot covered plug. (Lean is too white, normal is a brownish color)

Make sure the top (upstream) O2 sensor has less than 100k miles on it.

 

After reading this post it could be anything. Looks like it's time to park this car and wait for tax return time because it's going to cost an arm and a leg to get all these issues taken care of. This so far is what I need: Fuel pressure regulator, new IAC, New coils, New O2 sensors rear and front, catalytic converter, fuel filter, fuel pump. Geez just about $3,000 worth of work right there. I guess I had it coming for a 12 year old car that has cost me next to nothing in maintenance costs. When it rains it pours I guess.