hello, I was bench testing my 02 sensor off of a 90 5.8 E350 using one of the procedures from one of the websites. I'm a little confused on which wire to connect to, and why are some 02's are one, two, three or even four wire? They say to set a voltmeter ground probe to case and pos test probe to hego signal wire. My first question is, does the pos. probe go to the signal return (pin #29) or signal ground (pin #46)? My second questions is why does the sensor need to be heated with the other 2 wires? thanks for any response

 

Positive probe would go to pin #29.(darkgreen/purple).

From what I understand, the oxygen sensor is sort of like a battery. You have two types of metals in there, that when they get a certain temperature, some sort of chemical thing starts happening, and it generates a small voltage in proportion to the oxygen content of the exhaust. None of this will happen till it gets to a pretty high temp. While the engine is warming up, the computer does not look at the oxygen sensor. It makes decisions based on data tables stored in the memory. This is not the best for emissions, so they want to warm the oxygen sensor up as quickly as possible so the computer can use it for fuel decisions rather than the memory tables.

 

Also, headers/exhaust pipes lose heat more quickly than cast iron manifolds. Most GM (older ones at least) systems use non-heated sensors, but recommend changing to heated sensors if swapping to headers. If you shut off a vehicle for a short while and restart, the coolant temp sensor will tell the PCM that the engine is warm and allow it to go to closed loop right away, but the O2 may have cooled too much to operate properly. I thought since my '94's sensor was mounted in the downpipe that was why it was a heated unit, but have since seen several cast iron manifold sensors on other Fords that are heated as well, so maybe it's just because they want to get the system into closed loop as soon as possible. Also, on OBD-II systems there's another sensor after the converter, which is WAY downstream of the actual exhaust port, and even though the converter gets hot, it may not be enough heat to keep the sensor properly warmed up.

 

thanks frank2 and 2f150, you both gave me a better understanding on the O2.