Where are the oxygen sensors on a 1999 GMC Suburban?

O2 sensors

There are 4 sensors (2 pairs) with completely different connectors so as not to get them mixed up. The first two are monted just below the exhaust manifold connection on the pipes. The others are located near each of the two catalytic convertors just behind them on the pipes, the forward one points toward the outside frame rail & the other sensor points toward the driveshaft. You should be able to see all of them if you slide under the vehicle.