Hi All,

I picked up a '03 Reg 2.0L Mazda 323 last week. it had no issues at the time but around 100 miles later it popped up an error code (believe P0038). I cleared it and asked the old owner who said he had the same thing, cleared and it had not returned since. it did not come back for another 50 miles when I cleared again.

I drove around 100 miles after this and the engine warning light started flashing and I lost power fast. I pulled in, waited, and then was able to continue back. At this point I had 3 errors - P0443, P0300, and P0038. A mechanic had a look and pointed out a raised coil pack with a small crack. I got this changed to a new one and at the same time changed the spark plugs and cam belt.

Around 150 miles later the engine warning light came on. The car still drives smooth and this time the code is P0421. A google indicates it could be the cat or a sensor, could someone confirm how I might be able to check which one?

It's still driving very smoothly with no high idle or roughness at all. i hope it's the O2 sensor as obviously this is a much cheaper part and I've already put a fair bit into the car.

All help very appreciated!