Misfire codes

[s80newb]

Hey folks,

I have a 2001 Volvo S80, 2.9L (non-Turbo).

I replaced the upstream O2 sensors (the ones closest to the exhaust manifold) 2 weeks ago. Car ran fine until a few days back.

It started vibrating a bit at idle. The vibration goes away at higher RPM. Code scanner gave a P0304 which is a misfire on cylinder #4.

First, I replaced the spark plug with a new one. Nothing changed.
Next, I switched the #4 and #5 ignition coils, hoping, that after clearing the code the computer would spit out a P0305 which would indicate the ignition coil was bad since the error code followed the coil.

Unfortunately, I'm now getting a P0300 which is a Random misfire/unknown cylinder.

Anyone have any idea what could be going on?

Thanks,
Adam

[Livesound2]

As I understand it, if one ignition coil has failed the system is working harder electrically to absorb the problem. The remaining coils resistance changes.
Install all new coils and plugs if car has over 150k on it.

Hope that helps.

[im2sxe4]

Could be a blocked Cat. The one in the back not the two in the front.
If any of the cats in the front blow, they clogg the one in the back.
Ignition coil codes start popping up and you cant really drive for long b/c the car shuts off

[regent]

I would look at the fuel trims first just to rule out fuel/air issues before throwing more (expensive) parts at her. If you've got no VIDA/DiCE, you could see the trims with a generic OBDII comm. tool (Torque or similar)