Help - P0420 on OEM downpipes

[silverfoxS65]

Hey all. Looking for some help here
Car used to be FBO BM3 STG 2 with catless DP. Recently put stock DPs back on and flashed back to Stage 0 for annual inspection

Well I keep throwing a CEL. Cat below efficiency threshold.
Car only has 60k and DP maybe have 53k on them.
I replaced the post cat o2 sensor
Other things that can cause this?


I'm really trying to avoid replacing OEM DP for just 1 week for inspection.

Thanks in advance

[Dwaynos]

Originally Posted by thatorangem5

Hey all. Looking for some help here
Car used to be FBO BM3 STG 2 with catless DP. Recently put stock DPs back on and flashed back to Stage 0 for annual inspection

Well I keep throwing a CEL. Cat below efficiency threshold.
Car only has 60k and DP maybe have 53k on them.
I replaced the post cat o2 sensor
Other things that can cause this?


I'm really trying to avoid replacing OEM DP for just 1 week for inspection.

Thanks in advance

You really need to have the voltages checked. An experienced technician will be able to tell you what the problem is by monitoring these on a scan tool or via a scope.

I think the m5 uses wideband sensors for upstream and narrowband sensors for downstream. If this is the case then the pre cat sensors will show current (amps) and the post cat will show voltage.

The post cat is the one that will tell you what your cats are doing but they need to respond within a reasonable time. It takes a few tests to find out whether its 1 cat or both, again the technician will tell you which cat is at fault.

Also check that the cats are sealing against the turbo flange properly, although I'm sure you would hear that. The tech would see that through data aswell.