obd code p0305

Hi, im new to the forum. I have a 1997 blazer. This morning the check eng. light came on. I checked the code and it is p0305 (cyl 5 random misfire). Could this be caused by a bad catalytic conv. The cat has been rattling for some time and seems to have gotten worse lately.

Hi, im new to the forum. I have a 1997 blazer. This morning the check eng. light came on. I checked the code and it is p0305 (cyl 5 random misfire). Could this be caused by a bad catalytic conv. The cat has been rattling for some time and seems to have gotten worse lately.

Welcome to AF.

Code 305 is only caused by a missfire on no 5 cylinder.

What engine and how many miles.
Any flshing check engine lights driving down the road?
Can you feel it miss under load?

How are spark plugs and cap and rotor and wires?
Good Fuel pressure?

the eng is a 4.3 l. there are 155,000 on it. the CEL light came on once and was flashing. cap, rotor, wires, and plugs were all replaced 50,000 miles ago with ac delco parts. fuel pressure is good. there is a miss at highway speeds. also it sounds like the cat is plugged.

the eng is a 4.3 l. there are 155,000 on it. the CEL light came on once and was flashing. cap, rotor, wires, and plugs were all replaced 50,000 miles ago with ac delco parts. fuel pressure is good. there is a miss at highway speeds. also it sounds like the cat is plugged.

How good is fuel pressure.
Give us the readings on road hot or at time of missfire.
If not 60/66 on the road hot fuel pressure is low.

How does the cat sound plugged?
Check vacumn reading at idle and at 2K rpm for restricted exhaust.
if running vacumn is lower than idle vacumn you have a restricted exhaust system.
Post back vacumn readings.

If light flashing and code 305 check cylinder 5 for bad plug wire or dist cap and rotor.

Los fuel pressure and bad maf sensor can also give missfire but will usually be a ramdon miss on more than one cylinder.

Any after marrket alarm or remote starts on it?

I will have to check the pressure and vac readings tonight or tomorrow. the exhaust was making a hissing sound when revving the engine. Yes it has a remote starter on it.

There is a GM TSB on remote starters or aftermarket alarms that can cause a missfire problem..

sorry it took so long to get back to you. i changed the cap, rotor, wires. and the cat. converter yesterday. #5 wire was rubbing on the throttle cable, which may have been causing the missfire. the old cat was broken inside and part of the honeycomb was turned sideways in the cat so it was about 80% plugged. car is running great now. thanks for the help.

sorry it took so long to get back to you. i changed the cap, rotor, wires. and the cat. converter yesterday. #5 wire was rubbing on the throttle cable, which may have been causing the missfire. the old cat was broken inside and part of the honeycomb was turned sideways in the cat so it was about 80% plugged. car is running great now. thanks for the help.

You are welcome and thanks for posting back how it went and the fix.
Good luck