I think you can get to the O2 sensor from the top but am not 100% sure. Best to take the air box out, have a look and make your own decision based on the tools and experience you have. The light may extinguish after a few drive cycles but if not then you will have to have the codes cleared.
For info, P0420 is in fact a catalyst efficiency code, this could be caused by a failed lambda sensor or it could be a failed catalyst.
This should be a temporary engine management fault but I am not certain of this. If the lambda sensor is to blame, fitting a new one that works should clear the fault when the ignition is turned back on. Your car has onboard diagnostics so the codes should differentiate between a precat lambda fault and a catalytic converter fault. Older cars without OBD were a problem as high emissions could be caused by a fault in either part and it was a toss up as to which needed replacing.
I found this link which explains in greater detail…
The cat should have at least a 12 month guarantee - the ones I sell have 24 months minimum.
If your car has had any kind of fault that would allow unburnt fuel into the system, a new cat can be killed in a matter of weeks. Part of the problem with cat replacement is to be sure that the original has expired due to old age rather than been killed off by another fault. Hopefully this matter will be easily resolved.
The Cat was purchased through MX5 Parts and was purchased with a new mid pipe and dual exit rear silencer. The later was removed and replaced by a brand new standard system prior to selling the car to Colin. The original Cat was not faulty and was sold to another member on here whose cat had failed the MOT.
Colin’s fitted the new sensor but the fault FI light is still being thown after being reset.
Just had my first experience of a Mazda main dealer - total waste of time
I think I knew more than they did and I have only had an MX5 for 3 weeks
Basically they said it could be a whole list of things and that the code is misleading
Guess what - book it in and we will just work through it trial and error - not an exact science is it
So to summarise we believe the Cat to be good as per Dals post, the front sensor has been changed and each time after 2/3 runs the light comes back again with the same code P0420 Bank 1
Basic checks first. The sensor wires should lead back to a connector at the front of the cam cover. Was the replacement sensor genuine Mazda or an aftermarket part? Did you fit the harness supports which bolt through the exhaust heat shield to prevent physical damage? First check is for physical damage to the harness. At the connector on the front of the cam cover there should be four wires 1. Black/white. this is the 12v supply to the heater. 2. Black/red is the sensor earth ( terminal 4 O in the ECU.) 3. Blue connects to terminal 4W at the ECU 4. Black/orange connects to terminal 3J at the ECU. Then you really need an oscilloscope to check the switching voltages and the frequency of the switching. A local diagnostics specialist may be the best way forward here. Do not rule out the catalyst or the rear sensor. Ideally bothe sensors need testing at the same time to analyse their performance. If the rear simply follows the front it would point to a defective catalyst. ( Despite it being new etc.) Without thorough analysis you are simply stabbing in the dark unless the cause is obvious, i.e. physical damage.