if throwing a code from possibly being to lean, that could mean that your rear 02 sensor could be fouled. maybe try the CEL fix DIY for that. do you have a test pipe or high flow cat? maybe even a down pipe could contribute to fouling that reading.

 

Update to my rough running evo 8 (Buschur TB and Exede only) throwing a PO300 code. I went ahead and made an appointment at the Mitsu dealer just in case my attempts at repair failed. The service manager said he would run a diagnostic and do an ecu service update for $95. Well, yesterday I put in a full tank of good gas with a bottle of Techron and removed the Exede unit which I had been running for almost three years. After doing the above it cleared the code (you unplug the ecu under the glove box to remove the piggy back Exede). The evo ran great, no codes, no rough running after 10 miles of driving. All is well, .........not. I figured I would cancel the Mitsu appointment on the way to work this morning. Well, I take my kids to school this morning and after about 6 miles of driving the service engine soon light comes on. I drop off the kids and head to the dealer for the appointment. They called a few minutes ago and said it was throwing a PO420 code (catalytic converter) and nothing else. They did the ecu update and cleared the code. Obviously, they don't have an aftermarket Bushcur cat sitting around so I have an email into Buschur to see if they can sell me the cat ($150 list) for anything less than that.

 

try this, this might fix your problem

 

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Sounds to me like all you need is the mechanical o2 fix...please see this thread ($5 fix)

https://www.evolutionm.net/forums/sh...mechanical+fix