P0301 Cylinder Misfires and P0420 Catalyst System Efficiency

[kngky1002]

Hi. Still new to this forum so I apologize if there is already a thread about this. I have a 1998 Honda Civic EX Coupe that I bought 3 weeks ago. I recently got CEL and the scan said:

P0301 Cylinder 1 Misfire
P0302 Cylinder 2 Misfire
P0300 Random Misfire
P0420 Catalyst System Efficiency

Engine didn't start 2 weeks ago and replaced the dist cap but was unable to change the rotor because the screw had been stripped. I have the new rotor which I will replace tomorrow now that I've bought a screw extractor. The metal on the rotor is corroded which is why I'm replacing it. The wires are also brand new. Also, I'm planning to replace my spark plugs tomorrow as well since I don't know how long the ones currently in the car have been there.

How do I go about diagnosing/fixing the OBD II codes. I feel like the misfires are being caused by the corroded rotor but I have no clue about the P0420. Any help would be appreciated. Thanks!

 

[lethal6]

A code P0420 may mean that one or more of the following has happened:

- A damaged or failed oxygen sensor (HO2S)
- Oxygen sensor wiring damaged or connected improperly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
- Retarded spark timing
- The oxygen sensors in front and behind the converter are reporting too similar of readings
- Leaking fuel injector or high fuel pressure
- Cylinder misfire
- Oil contamination of the catalytic converter

Start with the obvious and easy ones to check and go from there. If you aren't mechanically inclined, I would suggest taking it to a mechanic to diagnos. Once they give you the answer, you can decide weather you want to replace what is wrong. I would NOT suggest throwing parts at it without finding out what is wrong first. The main thing people do with a P0420 code is throw an o2 sensor at the problem which is wasting money if you are unlucky and it doesn't fix it.

 

[RonJ]

After you do a full ignition system tune up (plugs, wires, cap, rotor), reset the ECU and run the engine to see whether any codes return. If P0420 returns, you'll need to replace the cat.

 

[FlatRate]

Can the car start at the moment?

Most times if there are codes for misfire and a P0420 cat code it is most likely due to the cat. Do you currently have any performance or driving issues? Like maybe low power, loss of power, can't get to certain speeds?

If the code P0420 stays on long enough it may cause a clogged cat causing your exhaust system to not be able to get rid of all your exhaust produced from the engine. It then backfires it into the engine causing those misfires. Easiest thing to determine this is to put a vacuum gauge on the intake manifold and see what kind of vacuums you have at idle and at about 2500rpm or 3500rpm in neutral.

If you don't have a vacuum gauge and the car can't start or has very low power to move even with the throttle to the floor, I would remove the front O2 sensor and see if the engine can breathe. With the O2 out of the way the exhaust can escape and engine will run better. Not like it should but better if the cat is clogged because the opening for the O2 is very small compared to the size of the factory exhaust.

Those would be the way I would look at it diagnosing your issue. Hope some of it goes to good use and makes a difference.

 

[kngky1002]

Thanks for the help guys. I dont have a garage and its freezing here right now so i havent gotten around to the ignition tune up. I'll work on it soon as the weather gets better and update you guys. Thanks

 

[kngky1002]

So I replaced my spark plugs and did an ignition tune up and then restarted the system to get rid of the CEL. After about 1-2 weeks of driving the CEL came back and now the code is just P0420. So you're saying I need to replace my cat now?

 

[RonJ]

Yep, if the misfire problem is now fixed, it's now time to replace the cat.

 

[sleeper si]

or if you are a cheap arse you can buy a 3 dollar spark plug anti fouler to get rid of that CEL.