Further tests;

After disconnecting the negative battery terminal for 30 secs, the
codes seem to be reset. The diagnostic codes are 11111111111111111...

The behavior at the exhaust pipe/oxygen sensor is the same - an initial
reading of 0-100Mv and no exhaust pipe smoke rising to 850-950Mv with
smoking.

Disconnecting the oxygen sensor does not change these symptoms and does
not change the diagnostic code which is still 11111111... No overall
engine warning light comes on.

Why does disconnection of the oxygen sensor not change the diagnostic?

Thanks for any ideas.