Use a 'scope. On a hot O2S, the switching time ("rise time, fall time"
if you're into EE) on a high speed idling engine for a zirconium dioxide
oxygen sensor should be under 100 milliseconds. If the sensor's output
lead's close to the ignition system, you may notice hash on it (e.g., on
my 1995 Ciera with 3.1 liters engine, there's plenty of hash -- scan
tool intermittently shows cross-counts of 255 -- this occurs only when
O2S output voltage > 450 mV).
If switching time's greater than 100 mS, raise the engine rpm. If it's
still greater than 100 mS, replace the oxygen sensor.
Oscilloscope type doesn't matter. I originally used a 25 MHz dual
channel Heathkit analog scope; now it's a Tektronix THM465
"etch-a-sketch" DSO (both are obsolete). You don't need anything that's
high bandwidth (this isn't Schottky TTL or ECL circuitry you're
troubleshooting or designing).
Regards,
Bohdan Bodnar