Hello Friends,

Im a new guy here and I have a 2003 Mistubishi Galant ES, 4 Cylinder with 104,000 miles on it. Recently I had error code P0421 on it, which apparently is the most common issue for Mitsubishis. I spoke to a local mechanic and he said that this model has 2 catclytic converters (one at the bottom of the car and the other in the hood near the engine). He suggested me to change the bottom one as it is cheaper and see what happens. I got that done for $160 and the service engine light came back ON again in just two days. Hence, he now says that the problem is with the catalytic converter in the hood and would cost me $550 to replace.

I wasnt convinced with what he said and I took my car to another mechanic and he ran the OBD and said that the P0421 is for the conveter at the bottom and told that the first mechanic dint replace with the right one. He also said that P0421 is the error code for the converter at the bottom and P0420 is for the one in the hood. Now I am totally confused whom to believe, someone please suggest me what I shoud do and also I highly appreciate you guys if you can throw in some light on which converter to be replaced based on the error code P0421. Thank you very much for your time.

 

I have the same code on my Galant ES. I was told it refers to the rear oxygen sensor.

 

Thanks aplustool. So does that mean I should just change the sensor but not the actual catalytic converter?

 

Thats what I did. My problem was also a bad idle problem. That cured my problem. I talked to a muffler man about it. I was getting 27 mpg and no lose of power. He said it did not sound like the Cat was bad. I also unhooked the tailpipe from the first Cat along with the O2 sensor. I shined a small flash light into the hole and looked in the other end. It was all clear. I have 180000 miles on the car.

 

Thank you very much, I will talk to my mechanic about it. Appreciate your help!!

 

Proper way to check it is to connect scanner and look at live data
I had a few cars I had to replace converters and out of 10 times aftermarket did not work,had to go with OEM and re flash ECU
Couple of times found by looking at live data lazy O2 sensors, just to old and tired,replaced them without replacing converters

 

Thanks Glimerman, I appreciate your time and help!!

Best Regards

 

Chris68, Did you get your problem resolved? I have 198,000 miles and am having idle issues with no code. My brake pedal is now sticking in the up position sometimes.

 

p0421 code....Warm Up Catalyst Efficiency Below Threshold

 

I own a 2007 Mitsubishi Galant 2.4L ES that's doing the same thing I changed out both downstream O2 sensors replaced O-ring gasket on the flange pipe, and replaced the exhaust header gasket and it still popping the code P0421. I'm adding a new muffler in hopes of going away. I need the check to stay off long enough for me to get it smogged so I can get the car in my name and registered.