>
> I'm a bit suspicious of the O2 sensor, is there an easy way to confirm if
> it's OK? What does ECU do when O2 sensor is unplugged?

ECU is perfectly happy running without an oxygen sensor.  All the T3s
delivered in Germany ran that way until they came to the States.  The CAT
is the only thing that cares about it.

Unplug the single sensor wire from the bright green ECU (shielded) wire.
Verify 0.45V to ground on bright green wire with key on.  If zero or near,
push back the connector boot and repair the broken insulation where the
shield is cut back, allowing stray strands from the shield braid to short
to it..

The ECU uses that 0.45V as a probe -- when the sensor gets hot enough to
run it overpowers the extremely weak (high impedance) signal from the ECU.
ECU sees the voltage changing and believes the sensor has come on line.  If
the line is grounded then the ECU will shift into closed-loop mode a minute
or less after starting (unless the engine is very cold) and will make the
mixture very rich trying to get the sensor voltage to rise.

Exhaust leak or misfiring cylinder will also confuse the sensor and force a
rich mixture.  And the sensor itself can fail.

Yrs,
d