Using a multimeter as a cheapo scangauge?
 

I've been monitoring my O2 sensor in a 4 cylinder 4x4 truck. Seems that it has difficulties staying in closed loop.

For whatever reason...any real throttle to speak of causes the O2 to read high and stop cycling (goes into open loop).

In 5th it will cycle at around 35 to 45 mph and from maybe 60 to 65 mph. Around 55 mph it usually won't unless warmed up real well...after a 20 mile cruise.

If I "feather" the throttle carefully I can usually get it to cycle...or maybe I have to drop it down a gear into 4th.

Not sure if this is the older "tired" O2 (have a new one to replace it with) or a sensor such as the throttle position sensor.

I'm assuming that if the O2 cycles most of the time...my mpg will be better.

Using a digital multimeter...any O2 can be monitored if you run one lead to the O2 to computer wire and one to a ground. You'll want to avoid shielded O2 wires though. Put the meter up on the dash and watch the action.

The multimeter idea won't be that good as a Scangauge... However, it could be of great benefit for people who are using heavier loads on the engine for pulse and glide - because you could see when the engine moves out of closed loop and enriches - and therefore you can apply the throttle to just before the point where it goes out of close loop!

Excellent idea for us with pre-OBDII cars and limited budgets. If nothing else, planting the wires in place gives you an easy way to check the O2 sensor periodically. Or I might get fancy and put a nice LCD gauge on the dash somewhere for continuous display (can't use analog for this due lack of resistance in the gauge).

For anyone wanting some of the tech background to make this work, see https://www.mr2.com/TEXT/O2_Sensor.html
It gives a nice review of the operation and testing of the O2 sensor.