to test em, look at the 3 wires coming out of the plug. there should be 2 fattish ones, there for a heating element inside it, and i thinner one. make a slit on the thin one and put a multimeter on it and check voltage.
lean is low voltage 0-0.1
rich is high voltage 0.8- 1 volt
mine is sitting on 0.01 volts all the time, which makes the computer think its running lean, so it pours in more fuel. hence the big black spots on my drive, and clouds behind my car.
also it should read .85 volts for the first 40seconds just after starting. not sure why that is.
btw got a 02 sensor and temp sensor ordered at work today, cost $74 very happy!
1976 351c td cortina