O2 Sensor failover???

[RawbDidIt]

Yes. B1S1 and B2S1 control fueling. B1S2 and B1S3 are to monitor catalyst efficiency.

Interesting. I figured it was the same exhaust system between the two of them, so it would use upstream for fuel trim and all four for cat efficiency, didn't realize the 305 only had one cat.

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[Motorsports-X]

So the 305 has 3 upstream o2 sensors?

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Correct. 1 for each manifold, and 1 in the y pipe pre cat b1s3 post cat.