P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
P0430 - Catalyst System Efficiency Below Threshold (Bank 2)
P1131 - Oxygen Sensor Engine Lean (Bank A) Upstream Catalytic Converter
P1151 - Oxygen Sensor Engine Lean (Bank B) Upstream Catalytic Converter

My 2001 has 140k miles on it; should I replace the entire exhaust system, or just the cats? Is it worth changing all the O2 sensors at this point, or just the two upstream that are retuning faults? I had a P0133 (slow responding HO2S) last week, as well.

 

If O2s on both sides went bad together I'd be looking for a common cause (wiring, fuse, relay, ...)

Otherwise just replace the upstreams and see what happens.

 

Clear the codes and see what comes back. As JagV8 said it's hard to believe both side would go bad at the same time?
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