UK Auto Repair Questions?
Hi there - I would unplug the sensor, and take some readings with a digital voltmeter - no analog meters for modern cars.
Expectations with the sensor unplugged - 2 wires are for the sensor heater - a power and ground - probably should see system voltage on one wire, ground on the other. The other two wires go to the computer - one should have a bias voltage of 450mv (0.450 v) and the other should be a ground back to the computer.
Hope that helps. Canuck