Check eng lite is on with a code of P0420 which is Catalyst System Efficiency Below Threshold (Bank 1). Looked at some of the old threads for the code already.

My Code Reader can also read while engine running and I'm trying to determine if the problem is one of the 02 sensors.

This is a 2008 H3 I5 Engine w/approx 105k miles.

With engine running at idle I get the following readings in Bank 1:

O2 Sensor 1: Voltage fluctuates from 0.080 to 0.805
Fuel Trim fluctuates from -1.6% to 4.7%

02 Sensor 2: Voltage fluctuates from 0.680 to .750
There is no Fuel Trim reading

Wonder if anyone with no trouble code issue and a diag tool that reads while engine running can hook it up and tell me if they get similar readings to compare with mine. Also, whether you get any Fuel Trim reading from Sensor 2.

I suspect that may be the faulty sensor unless it turns out to be the catalytic. Not sure if its normal not to have the Fuel Trim reading in Sensor 2 or if this is the indication that its defective.

Any help would be greatly appreciated.

Thank you.