I have a 2003 S60 AWD with 105,000 miles, and I drive it back and forth from Seattle to eastern WA weekly. A few months ago, it started throwing the P0303 code, and when it did, it ran extremely rough, but would then clear up inexplicably. Since it started, this problem has ALWAYS been intermittent, meaning it would run for days or weeks with NO problems, and then this would show up, sometimes only once, sometimes on and off for days at a time.

I've replaced the #3 coil and plug, it worked well for about a day, and the problem came back (again, intermittently). As summer sets in, it seems to be getting worse and more often. On Sunday, I took it to O'Reilly and pulled not only P0303 but also P0203. I pulled apart the injector rail and switched out the #3 injector for the #5 (I think it's #5 - the one farthest on the passenger side). I also swapped the new coil from the #3 cylinder to the #4 cylinder. I test drove it, it worked fine, and then Monday when I drove to eastern WA, it started to run HORRIBLY. I pulled into an Autozone and was bombarded with the following codes: P0201, P0203, P0300, P0301, P0303 (now that I think about it, I bet the passenger-side cylinder is #1 not #5). It ran awful, but would clear up under high revs. It also cleared up completely when the car sat in the shade for long enough to completely cool down.

Today I got in the car, it's been rainy, cloudy and in the 60's, and the car runs with no trouble, no rough idle, nothing except that the CEL hasn't cleared yet.

Autozone said the possible fixes for all these codes have included "Replaced Brake Booster Vacuum Pump" and "Repalced Brake Booster". The fact that this problem is more noticeable and constant at higher ambient temperatures indicates a vacuum problem to me, but why the brake booster?

Any thoughts? I've searched the forums, found tons of threads about the P0303 code, but nothing that talks to the intermittent nature of my problem. Why would it come and go like this?

Thanks!!!

 

Don't let AutoZone diagnose your car.

Those have nothing to do with the brake booster.

The injector codes aren't common and neither are injector failures. Make sure none of the wiring to the injectors or coils is pinched, chewed or otherwise messed up.

Otherwise its either bad injectors (1 and 3) or a bad ECM.

 

Here's an update on this: The car continues to run poorly MOST of the time, but other times it runs without problem. I did check the voltage on the fuel injector signal cables, and found that while all 5 injectors showed ~10 volts when running, when I shut the car off, #3 (the problem cylinder) voltage dropped to nothing, while the other (4) signals dropped to 8 volts.

Does that point to anything? Where do the fuel injector cables come from/go to? It gets a signal when running, but nothing when not.