Which o2 sensors seem to b the 1s to fail??

Replacing the oxygen sensors just because is a waste of money. Oxygen sensors last a very very long time. More often than not the life of the vehicle! But if you decide to replace them never buy a universal oxygen sensor, no matter what brand it is. The wires have to be installed in one of the connectors provided then crimped together. They're a pain in the neck to work with and often fail after a short time. The upstream oxygen sensors monitor the oxygen level in the exhaust coming out of the engine. The signal is sent to the engine computer to maintain the air to fuel mixture at 14 to 1. The upstream oxygen sensors usually fail as a result of an engine problem. The downstream oxygen sensors monitor the catalytic converter, nothing else. The signal from the downstream oxygen sensors to the computer should be very close to a flatline. If the signal begins fluctuating too much that tells the computer the catalytic converter is not working properly. Hope that helps! Jim

Once one goes...they all go, unfortunately. Best to replace all at one time and avoid labor costs in the future.