: 96 Deville o2 Sensor question

03-19-04, 04:51 PM
My check engine light came on, reads PC0139 current, im told thats the upstream o2 before the cat, i looked at some websites to get some prices and they all say i need 2 before the cat, i looked under my car and i only see one o2 and one pipe, the exhaust doesnt split till the very end. Whats the deal? Thanks James

03-19-04, 05:43 PM
Bank 1 Sensor 2 is directly in front of the cat-con. Recommend OEM parts only for compatability.

03-20-04, 12:48 AM
There are 4 O2 sensors in the system. One on the left bank, one on the right bank, one immediately before the cat and one after the cat. Replace the one immediately before the cat. Some resources will say that there are two or even three O2's before the cat referring to the left bank and right bank O2 sensors as being "before the cat"...the real pre-cat O2 sensor that you are looking for is the one that is a few inches ahead of the cat in the pipe under the car. Use the OEM O2 sensor for compatibility with the OBD2 diagnostics.