[Andystobbs]

Haven't read whole thread fully, but I once had a lambda sensor short internally. From signal to heater power. Was especially confusing at the time because the diagnostic platform (this was on a Vauxhall) would only display upto 4.99V for signal voltage, despite the wire actually being at 12v.

I'm saying this because its not impossible to imagine a signal line internally shorted to heater ground, which would give intermittent 0v, or sensor ground giving permanent 0v.

Ps. The pre cats look to be wideband sensors. They operate differently to be able to interpret not just rich or lean, but how rich or lean. They do this by measuring the current requirement to pump oxygen from one side to the other.