I installed a full Miltek exhaust on my 05 MCS and I keep getting the code “Catalyst Under Threshold) I clear it then comes back 4 days later. What a nuisance!

 

Try an 02 sensor defouler with it

 

Sounds like you have a prob with the O2 sensor. They're fairly easy to damage. Is it possible that it was knocked during install?

 

no it’s been replaced twice since the code.

 

I did then it caused a hard emission code and took a month to find the right equipment to clear the code so I removed it. Plus when I removed it the small catalyst inside was gone??

 

????
A defouler is just an L shaped metal tube that spaces the 02 sensor away from the exhaust mimics the reduced flow of a cat to trick ecu.did you install the front or rear 02 sensors into it ?
if you couldn't clear the code rite away even with a 2$ walmart scanner then the car was still detecting an issue and throwing the code no matter how many times you tried to reset it or it had to perform the proper drive cycles until it detected the problem is gone...I went through it all on my car with 2 codes related to my performance exhaust.
how old are your 02 sensors ? They may not be old and bad but they could be running lazy not fully throwing a defective 02 code but throwing codes related to the exhaust system not functioning correctly , when it us it's just the sensors on it's way out. This can also be the case with junk Ebay and amazon sensors too even if new.
do you have a cell phone and obd2 dongle ? You could always look at real time values n data n verify which sensors are running within spec.

 

The problem was I had to reset the adaptations and nothing I had could do it so I had to invest in a $1500 scanner. After the adaptations were reset I was able to clear the hard code so now every few days I get the nuisance code. The rear 02 is less than a month and the front a few months old. Maybe the defouler I bought was garbage and need a good quality one. What do you suggest?

 

I bought one off Amazon that I used on multiple vehicles . It is adjustable in lengths to get the right reading. You also need to make sure its angled correctly or the condensation in the exhaust messes it up.

 

Fault code P0420 occurs when the catalyst is removed.Right fault code name is "Cathalyst system efficiency bellow threshold".Easy way is to do this fault code off.
I can help you, free charge if you can read the DME software with any cheap read and write tool,available on the internet for not more than 60$(MPPS V18 is a good tooll(compatible with Win 7,8 ,10) to read and write DME sw. Need to read DME, send me the 512kb file , I'll modify it for DTC P420 off and send you back.Just need to write DME with new sw version.