Another P0420 CEL

2002 Protege 5/ Manual transmission
Well, I've been trying to get my P5 back on the road. Started driving it again last week (first time it has been driven since April 2018). Put about 50 miles on it when the check engine light came on and started flashing. Misfire and dead downstream O2 sensor (had mechanic friend check it out). New O2 sensors (went ahead and replaced both since they are old) and swapped coils from my old engine and it seemed to be running well (best since getting car running again).
30 miles later and the CEL is back on solid this time. Car is still running very well. The code is P0420 but I'm getting now misfire codes. I have since put on 200 miles (2 days to work). Checked again and the only code is still the P0420.

Options as I see them.
  1. Replace the Pre cat.
  2. Replace the pre cat with salvage item.
  3. O2 sensor cheat mod
  4. Headers and O2 sensor cheat mod

I can probably handle all of the options except the new header myself (hopefully not wishful thinking but I do have large hands).
If I go header route, any tips/suggestions and are they worth it from a power/efficiency standpoint? I have always had a preference for quiet cars (although I love power and handling). Also, who still offers them?

Any suggestions?


IF it has been sitting for a year and a half, your gas is crap.

Drive it or drain the tank as much as you can. Ethanol gas separates into water as it sits, especially for that long. You've got water in your gas tank.

Maybe best option is to pul the fuel pump out from under the back seat and use a hose to siphon as much out as you can into a bucket. Then use some rags or paper towels on a claw tool to sop up as much as you can that's left over. fill with fresh gas and see how it does before you go replacing any parts.
2002 Protege 5/ Manual transmission
Thanks for the suggestion. We tried to get all of the old gas out before putting it back on the road. I've now put about 600 miles on it since the P0420 code came up. BTW, I have new coils and plugs in there since the misfire code.
Each day it seems to run better but the the code persists. Last tank I averaged 33mpg so I'll keep an eye on it for now. I think that value was not very accurate and once averaged over multiple tanks it will be closer to 30mpg. Back at 265K miles (when I let my daughter talk me into giving it to her) 31mpg took much more care driving to achieve than I am doing now.


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