O2 sensor codes

[scrubnick]

My SC400 throws a CEL intermittently while driving. It will come and stay for a bit, then go away. I checked codes and got 21, 27, 28 and 29, all of which are O2 sensors. I know the sensors were all replaced about 20,000 miles ago. Why am I having this problem?

 

[Peewee]

Is it possible that you are running lean, or severely rich?

Perhaps get a wideband o2 on the tail pipe and check that the air/fuel ratio is good.

 

[Nick M]

You could have a poor electrical connection. If the computer does not see the correct switching voltage, you will store codes. Just a thought. An exhaust leak will cause trouble if it is on the manifold.