In the IPD catalogue it says you can do a simple test. They do say it is less accurate than using a voltmeter.

Their test simply consists in disconnecting the sensor when the car is running. They say that the engine's iddle should fluctuate within 30 seconds and if you plug it back in it fluctuates again until it smoothes out.

Well when I disconnected the Oxygen sensor plug, no changes whatsoever in the idle.

I am just shy of 160K miles with original sensor still in. Should I assume it is bad? I am getting 18 miles a gallon suburban driving and I have done about 2000 miles ago

spark plugs and ignition wires
rotor and distributor cap
air filter / fuel filter
oil & oil filter
cleaned throttle body and control idle motor
flame trap
coolant thermostat

and just recently

power steering fluid
manual transmission oil
coolant flush

I am overdue though for timing belt and valve clearance -- don't think this would affect too much gas mileage, would it ?
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'89 244DL M47 158K miles