Well, there can actually be 3 of the O2 sensors.
The later models [like Arthur's Polo Green] that have OBD11 diagnostics, they run a more sophisticated system. It has the usual two sensors like we have, and an additional "after" to see how clean everything is burning up.
But Peter's engine is from and early JDM car, and would only have two O2 sensors.
Joe
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Black Betty [Bam a Lam!] '93 UK spec, still languishing
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