Hi
I got my MX5 last week and have to say what a brilliant drive. Should have done this a long time ago. Anyway, I am getting an engine management light which comes up after about half hour driving. I have a code reader which is showing P0421 Warm up catalytic converter bank 1-efficiency below threshold. Have researched this including on this site and cause could be either the cat not working correctly or an o2 sensor at fault. Further research suggests it is the post cat o2 sensor. As the fault is not occurring during warm up ie the first 5 or 10 mins I am leaning towards replacing the post cat o2 sensor. Have contacted the dealer that supplied the car and am waiting for a reply. Have told them that I have reset the fault code and am going to monitor but if it recurs it will need sorting. Any one else had this code.
Chasing your tail time here.
It could be a faulty pre cat O2 sensor, a faulty post cat O2 sensor or a cat at the end of its life. Youu can change all three or one at a time. It may also be something else all together.
If the car is still under a dealer warranty and some will not cover that system get the garage to sort it.
Report back if it is not covered but at the moment it’s a dealer issue.
Agree with Drumtochty - this is a fault code that points to several possibilities. As you are presumably covered by warranty, I would hope it is not your problem
If the diagnostic is unable to pinpoint the problem to a specific part, none of us will be able to either.
Thanks for the info. Thankfully it is covered under warranty and dealer is being very good about it. Have reset it and told dealer if it recurs it’ll need sorting.