various engine light codes

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133,000 miles and all the coils have been replace at least once, some twice. The O2 sensors have been replaced, the A/F sensors also. Recently a while after and ox sensor and a #2 coil was replaced the catalytic converter below threshold (bank 1) code came on. Toyota said replace the catalytic computer. ($1500.00) I kept driving it for 3 weeks until I was faced with misfire codes for #4 coil.. REplaced that coil. The next day the code came up for the recently replaced ox sensor. IT then disappeared and the car was fine for a week. Then the catalytic converter code came on again. Now isn't there something going on that could explain all of these failing parts...and how can I avoid the catalytic converter replacement which seems like a stretch both diagnostically and financially. thank you...susan