VersaTune User Forum

2014 MX-5 PRHT
Mods: Goodwin Racing Roadster Sport Street Header with catalytic converter, GWR Helmholtz mid-pipe, GWR Super-Q muffler, Track Dog Cold-Air intake

Versatune Basic tune
Began getting P0421 Warm Up Catalyst Efficiency Below Threshold Check Engine Light about 2 months after a basic ECU Flash
I cleared codes several times. After 4 months returned to factory flash and continue to get the CEL code P0421 after less than a week of driving

GWR is thinking over-rich fuel mix has burned up the catalytic converter

My shop cleaned Mass Air Flow Sensor and A/F Ratio Sensor (? - think that is first or upstream cat sensor) and cleared. Code returned in 3 days.

Thoughts?? Experience??
Is the CEL a result of the header? The ECU Flash (fuel mix)? Both?
Work arounds??

Pros or cons to:
replace O2 sensors on header - downstream first then upstream?
put 02 sensor spacer - basically a "pull sensor out of the stream" trick??
Use Versatune to turn off that Code test??
Replace the header to get a new catalytic converter??

Appreciate any help or insight. Bill

Are you running a spacer in the rear O2? This will help make the code less temperamental. Aftermarket Cats are more sensitive when it comes to setting Cat codes.

Sometimes there are some fueling tweaks that need to be made in order to keep the Cat warmed up enough to stay above the code threshold.

When you did all of the work, the car should have been data logged and verified that fueling was good. Sounds like this step was skipped.

Data log, diagnose and verify whats going on before throwing parts at it. If you are working with a shop, they should know how to do this. You don't need to keep waiting for the code to set in order to see if it's fixed or not..

Otherwise the quick fix is to use Versatuner to stop the code from setting the light. The code will still be present in the back ground, so if you need to pass emissions testing you'll fail.

Very helpful CC. Thks. Sort of learning my way forward, so this is helpful.

One other thing I forgot to mention. Just cause the code is setting doesn't mean the Cat is bad. It doesn't mean it's a performance problem, plugged or anything to worry about. It just means it's not passing a "subjective" ecu test. It's saying your car is putting out more than allowable emissions set by EPA..

0421 is absolutely normal with aftermarket high-flow cats. There is no fueling issue (you'd have codes for exactly that if it was the case) or any other issue. You have two options:
1. weld a spacer bung for your secondary O2 sensor;
2. disable the P0421 from versatuner when you flash the tune.