I had my check engine light come on. I went to the local murrays and used the obd2 scanner. It came up as catalist effiency below threas hold.

It cant be my converter because I changed that 6months ago and it is an aftermarket on.

I then purchased a bosch universal o2 sensor. I replaced the secondary sensor. I then pulled out the ecu fuse for 20 seconds then put it back in.

The check engine light was erased, then after 37miles of driving the light came on again.

What could be my probelm.

This is a b18c1 motor with a obd2 ecu.

 

It could be the cat.
Some aftermarket ones don't last as long as they should.

what brand is it?

 

The converter is a 2.25 inch magna flow. I got it put on last feburay.

 

Could it be your o2 sensor? Do you have a friend with a testpipe/cat you could borrow to figure out if it is the cat or the o2 sensor? Just swap cats with a buddy without the problem and if you still have a cel, it's not your cat, but something more. I would think magnaflows would last longer than a year.

 

If the code is P0420...then that means the O2 sensor downstream of the catalytic converter is detecting that the converter is not working as efficiently as it should be.

Since you have already reset the code and it came back, your next step is to check the voltage at the o2 sensor. It should be low (something like 200mV, but i'd have to look up exact spec to be sure).

My best guess in this situation is that it is the converter. Even though it's new, you said it is aftermarket and it could be letting more catalysts through than an OE one would, hence the code.

If it has failed, you may be able to call the shop that installed it. I do believe there is some kind of warranty on them. Especially since such a short time frame has passed.