CEL and P0140

My check engine light came on during the drive to work today, so I ran by Auto zone and had them read the code from it. Only had one code, P0140 which is "No activity detected" from an O2 sensor. I did some searching on the ::Batman:: board and found many threads stating it was the rear O2 sensor. In Jan/Feb of this year I replaced: Coil packs, plugs, plug wires, cat, and rear 02 sensor. The front sensor was changed by me in 2007, so I'm thinking that may be what is throwing the code. Is there any way it could be the front 02 sensor? I couldn't find any info to confirm this, it all points to the rear sensor. I'm going to crawl under the car tonight and check the wiring on the second sensor, is there any way to test a sensor?

Thanks in advance.

Maybe when you installed the rear 02 the wiring got crimped under the seat or burned on the exhaust pipe. I'd do a physical inspection first. I believe the front 02 is a different code completely. Also possible you either have a short in the wiring harness or just got unlucky and got a bad 02.

Super late follow up on this one, the off brand O2 sensor I got from Rock Auto was no bueno. Installed a name brand Bosch sensor and everything is fine now