William Kennedy writes: > Does this mean that an O2 sensor and a digital voltmeter make a crude emissions > tester? Anything else tricky in wiring up such a thing? Use 2-wire or 3-wire > sensor? Any particular unit easiest to start with? Anyone sell plans for > such a thing? > B. Keep in mind that the O2 sensor and any reading derived thereof are a measurement of the oxygen in the exhaust stream, not the CO. In general, a rich mixture will have little oxygen present and hence high mv reading on the sensor. The sensor produces the mv output signal from a differential between the oxygen in the atmosphere (there's a breather hole in the outside of the sensor) and the oxygen in the exhaust. More oxygen present in exhaust ie lean conditions=less differential and hence lower mv reading. So, it's not an 'emission tester' The three-wire sensors utilize 2 wires to drive a heating element keeping the sensor at optimum temperature, the third wire is the signal output line. Hook the DVM to the output line only. A single wire sensor doesn't have the heater built in and doesn't function properly until the exhaust gas warms it up to a critical level. Hook the DVM to the obvious (and only) choice!
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