Hello,

I'm driving a 2004 Lancer, with the 4G94 2.0l 135hp engine. The car has now run for 200k km without any issues, however the engine control lights are now constantly light up and the error(probably P0420, a friend of mine had the device to check what the system is indicating and told me it is the catalyst, so i assume it is this) is indicating an issue with the catalyst. Now as this would be a rather unpleasant and costly repair, I would like to know if anything else could be causing this error as well? For instance, the ignition plugs have never been replaced so far, as well as all the measuring units like the lambda sensor. I've also knocked against the catalyst to check for any rattling noise but i couldn't hear anything. On the contrary though, the idle seems to be a bit rough sometimes. Especially when it is still cold it jumps around for some time in the range of 1000rpm, although it is fine as soon at it gets warmer.

Even though i have no idea if this could have anything to do with it, oil still looks very good and fuel consumption seems to be in the realm of what is normal(around 7-8 liters when driving through mostly rural areas, this is a very rough estimate, i haven't kept close track of it so this might be wrong). We've already tried just to delete the error to see if it comes back up again and it did after 30km of driving. Talking about driving, it behaves completely normal when doing so. It accelerates like it always does, makes no issues at all.

So could any of this, ignition plugs, faulty sensor etc, cause this error to occur as well or is my catalyst dead/ needs cleaning? Thanks for all the answers.

 

P0420 usually means your catalytic converter has gone bad and needs to be replaced. If your friend has the tool to check, I'd find out which one for sure since there is two on the car; one at the exhaust manifold and the other under the car. In my case, the Lancer I bought used had a previous owner who took out the one under the car and just straight piped it, so when emission check comes up for me, I have to replace it, but my other one is also bad so I'll pretty much be doing a nearly full exhaust replacement from the engine all the way back. Another thing you can double check is if it throws it as the O2 Sensor. Typically, the reader will show what is wrong; whether O2 sensor or Catalytic Converter.

 

Bad converter.
May cost little something but not much on our cars they are easy too do.
just straight pipe it haha CEL eliminators will fix that problem forever haha