I replaced the ignition wires and reconnected a loose vacumn hose on my sister's 1989 Olds Cutlass Cierra (with 3.3L engine) last weekend. Engine performance improved right away from what it was. Now, according to my sister, sputtering and rough idle have returned. My dad thinks this might be a sensor, possibly the oxygen sensor. What do you folks think? Is it possible to test the oxygen sensor, or any other engine sensor, with just a multimeter?

Yes and no.....to be more specific, it depends on the type of multimeter used, and the type of sensor. A basic multimeter will test continuity and resistance in certain sensors, although on something such as an O2 sensor, a graphing meter is more suitable. Is it tripping a check engine light? How is performance otherwise. An O2 sensor switches voltage so quickly when it's operating operating properly that it's difficult to keep track of, although you should be able to tap into the O2 sensor wire with the positive side of the meter and ground the negative side to watch voltage changes, as long as the meter is digital. An analog meter simply will not react quick enough.

At idle your older car will drop back into Open loop at idle as soon as the O2 cools down and no longer switched, so yes it is possible it is causing the problem, but there are also SOOO many other things that could be causing a problem, clogged injectors, poor firing voltage, bad Coolant temp sensor etc, etc, etc, My best recomendation is too hook up a snaponMT2500 scantool and look for abnormal fuel trim adaptives at idle, open loop/closed loop operation etc.

BTW if thats the cars original O2 sensor it needs to be replaced anyway as a rule of thumb I tend to rec 50-75K mile O2 replacements or when tuning up the engine, an old sensor doesnt switch at the proper frequency and can cause fuel mileage to drop.

Good luck! James