hi there everyone im new on here just bought my first subaru getting it ready for inspection. . . .its a 96 legacy outback wagon and its throwing two codes. Th e first code is p0400 and the second is p0420. the car is running relativly well only bad symptom is a slightly low idle but nothing to alarming. since the codes are egr system exhaust recirculation flow and cat efficiency below threshold bank 1 could these both be caused by an exhaust leak between the engine and the cat because i suspect there is it certainly sounds like it? i just want to be able to set my priorities in fixing it and if the exhasut could solve these codes than i will fix that before replacing other components.

*Probably

I have seen intake leaks cause those 2 codes but this is more common on Impreza and Foresters from the 98-00 era that have two air filter boxes (well what looks like it).

Scenerio 1 is that the air filter box isn't engaged into the tabs on the bottom and allows an air leak.

Scenerio 2 is that some idiot has taken the air box spacer out and put a 2nd air filter on the car.

 

If you can get a copy of the 96 Factory manual it will tell you how to troubleshoot that code(s). I would start with the 400 as that is less common. I would also clean the EGR valve and verify all the hoses going to the EGR solenoid are in place and not misrouted or broken. That or get a new EGR valve. The 420 codes I fixed at the dealer I replaced the cats every time and never had a comeback.

What he said about he P0400.

If fixing the exhaust leak doesn't cure the P0420 code, search "P0420" on the site here and read up on the spark plug anti fouler trick. Simple, cheap, and almost guaranteed effective.