just got P0420 on 2001 toyota corolla.

I took my car to the mechanic (toyota dealership) to get an oil change last week.

The following things were done on my car:

1. Oil change.
2. New Air filter.
3. An oil leak was discovered, so he changed the timing chain tensioner o-ring.

2 days later, the check engine light comes on. The car has about 105k miles on it. This is the first time the light came on. I suspect it has to do something with the mechanic did. I took the car to an autozone: the code came back P0420--Catalyst system efficiency below threshold - bank 1.

Is there anything that could have been done to the car by the mechanic, , even by accident, that could result in this check engine light coming on? Could it be a loose hose or problem with the new air-filter causing the AF sensor to error. I highly doubt it is due to the catalytic converter, particularly since it occured right after I took it to the mechanic. I suspect it has to do something with what the mechanic did. Any help would be appreciated.

Regards,
Paras

I'd bet my next days pay it isn't related to what the tech did! The po420 code is very weird to comprehend for some in that one day the catalytic converter can just barely satisfy the pcm requirements for the monitor and the light doesn't come on because the rear 02 stays satisfied with it's readings and the next day or two, it runs it's tests again and what barely squeaked by now doesn't depending on fuel quality, temperature outside, soot on 02 sensors, exhaust leaks etc. Get it rechecked by the Toyota dealer and stay out of the zone for diagnostics