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!