The service manual for my 1996 GMC Sierra tells me that my truck has four oxygen sensors. They are designated: Bank 1 Sensor 1, Bank 1 Sensor 2, Bank 1 Sensor 3, and, Bank 2 Sensor 1. The manual does not clearly indicate the location of each of these sensors. A GMC technician tells me that the term "Bank" refers to a particular side of the engine - Bank 1 refers to the side with cylinder number 1, the driver's side (as the plug wires are numbered 1, 3, 5, 7). You'd think it would be easy to deduce the placement of each oxygen sensor. Here's the problem: Only one oxygen sensor lies beneath Bank 1. The other three are under Bank 2, two in front and one behind the catalytic converter. My information seems to be contradictory. Error code P0153 tells me I need to replace Bank 2 Sensor 1. Can you verify its location?
Somewhere along the line, you have gotten incorrect information. Whichever cylinder bank has three oxygen sensors must be bank 1. The cylinder bank with 1 sensor is bank 2. We would suggest changing all four oxygen sensors in view of the fact you have 70,000 miles on your vehicle. If one has failed, the others are probably close behind. Just a little preventative maintenance here.