Galant with multiple issues, P0421, P0300, P0128

[emcwill792]

Hi. I have a 2002 Mitsubishi Galant that is having some issues.

1. Autozone pulled codes P0421, P0300, and P0128
2. Engine runs rough through all RPMS - I'm used to Galants running fairly smooth, this is just way too rough in my opinion.
3. Oil light flickers on and off when driving, even though I have checked levels and they appear to be full

I have checked the ignition coils for the P0300, and they appear to be sending a spark.

Any suggestions? Please let me know your thoughts. I'm someone who is ok with repairing cars, although I do not have a whole lot of diagnostics and internal engine repair experience, so keep that in mind.

[emcwill792]

Looked around online this morning, and from everything I'm seeing the P0421 probably is my catalytic converter. Would that make my car run as rough as it does?

[varroa]

what are those codes?

[emcwill792]

P0421 - Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0300 - Random/Multiple Cylinder Misfire Detected
P0128 - Coolant Thermostat Malfunction

I have taken a video with sound so that you can hear the loud noise the engine makes:

http://www.youtube.com/watch?v=yLCUT-inr6M

Replaced the spark plugs today, no improvement. Please advise

[boochieboy]

The thermostat is a pretty simple and cheap replacement, the oil light flickering is almost definately the oil pressure sensor. The P0421 would be (I believe)your oxygen sensor(bank 1 is the sensor on the exhaust manifold I think).......somebody correct me if I am wrong....all pretty simple fixes. I would start with the 02 sensor(you may have to get an 02 sensor socket to remove it),,,and go from there,,,,,,,this would definately be a cause of a car running like shit. For the other code, you may want to check the wires and connectors for the coil packs.

[varroa]

what i don't understand is how can the o2 sensor BEFORE the cat determine that it's not warming up right.

regardless, that code description might indicate that you have an exhaust leak somewhere. try to "feel" the whole exhaust pipe from the manifold to the end of the second cat for obvious exhaust leaks. it will help if somebody is keeping the engine at like 3000 rpm at the time.

[emcwill792]

Would that make the car run rough/loud like it was in that video link? I'll be sure to check it as soon as I get home.

A little more information from when I looked at the car yesterday:
The old plugs seemed fairly new, but had a coating of black soot around the threads. Also, the fumes from the exhaust seem stronger than normal, slightly more "gas" smell than what I'm used to.

I did clear the codes yesterday and they did not pop back up when I was running it for about 10-15 minutes. Just seems to run very rough and loud.

[varroa]

why would you clear the codes before you fix the problems?

i don't think the exhaust leak would make your car run rough, unless it's huge. then your engine would sound like a riced-out honda.

problems with random misfire is probably either bad gas, or something wrong with the fuel line. all that WILL make your engine run rough.

[emcwill792]

I had cleared the codes after I changed the plugs, to see if they came back up, and the service engine light hasn't come back on.

I'll be checking the exhaust for leaks tonight and take it for another drive to see if the codes come back up.

Does the audio in that video give any clues?

[gizmo'sgalant]

from my current experience the p0421 pulled up a falty catalytic converter, exhaust leak or faulty heated oxygen sensor... my galant is an 03 and it runs rough too but my check engine light would come on and off randomly. i found it to be the heated oxygen sensor going out long story short.

[emcwill792]

Well, checked for leaks in the exhaust, and I haven't found any. Next suggestion?

Would a bad o2 sensor make the car run as rough as it is?

I really appreciate you all helping me with this.

[boochieboy]

You're assuming that it's just the sensor causing it to run that bad. It would make the car run shitty but it may be a combination of things....just do what I said and replace the sensor and see if it solves your problem. If it does, then move onto the oil pressure sensor, then the thermostat. Job done.

[emcwill792]

Replaced O2 sensor, and engine does not run any smoother - still rough and loud. Any further suggestions?

[emcwill792]

Is this to a point where I should take it to a shop to diagnose, or is there anything else I can do at home to figure it out? I do not know the previous maintance history of this vehicle.

[Hibolt]

P0300 I fixed it by changing the ignition coils (both) spark plugs and wires and that fixed that problem as for the P0421 its specifically talking about the 02 sensor closer to the cat not the one on the exhaust manifold but I changed both on mine anyways and that cleared that code also and for your P0128 cant help you there never had that one yet...

[emcwill792]

You know, just thought of something - could this be a problem with my balance shaft or balance shaft belt? Would that cause all the noise and vibration i'm hearing and feeling?

[emcwill792]

Damned thing was out of time - balance shaft and oil pump were both so far out of time it wasn't even funny. running it now, still a little loud, but runs smooth. First time working with the timing belts, wasn't half difficult.