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Re: Dangling Wires

> If the sensor is disconnected (or dead for that matter), the ECM will see zero
> volts from the O2 circuit. This is registered as a lean condition, and the ECM
> will respond by increasing fuel. This is the reason you should verify catalyst
> function, as an overly rich condition can damage it.

In the case of the MAC11, the O2 sensor input is biased to around 0.4 volts
by 2 684k resistors.  If the sensor is disconnected the ECU sees
a steady 0.4 volts.  If the ECU sees this value for a few seconds
and nothing it does to the mixture changes it, it sets the
O2 sensor disconnected/dead error and goes to a default mixture setting.