Many web sites say this can be done. I rigged up wiring so that my 91 Civic's one-wire O2 sensor output voltage was measured by my digital multimeter (impedance >= 10 Megohm) while still sending a signal to the computer. I drive the car 20 minutes, park it in idle, and let the fan come on, so I know it is warmed up and so presumably in closed loop. I check the O2 sensor voltage. I get very few crossings of the 0.45 V mark per minute; less than five is usual.

Should the multimeter show the high frequency crossings of an O2 sensor? From my reading there are supposed to be way more.

The O2 sensor voltage does do roughly what it is supposed to do when I raise the RPM (goes rich = voltage rises) and then release it (goes lean = voltage falls).

I am dealing with passing emissions, something I have rarely had to do in the past because of where I have lived. The O2 sensor is OEM Denso, five years and 55k miles old. I did not think this was very old for the sensor but now I see many sites say it is. I have been searching for causes and am now thinking that in fact my O2 sensor is dead. It is puzzling because the car runs great; though maybe slightly declined mileage from 40 mpg to 38 mpg. Hard to say because of a recent change in driving habits. There may have been some leaks at the throttle body gasket, even though idle RPM seems steady. I Hondabond-ed what I am almost certain was one leak (no more whistling) and think I glued over another today. My latest theory is the TB leaks caused a rich running condition. The richness burned out my O2 sensor over time (and maybe cat). Cat is new (Magnaflow direct fit) as of September.

 

Well thought through diagnosis. That oxygen sensor switch time is very slow. It really should be much faster. If it were me, I would go ahead and pull the trigger on the new oxygen sensor, you have your bases covered.

OBD1 cars, generally regarding oxygen sensors, are not really going to throw any type of check engine light either unless the sensor is unplugged or there is an open circuit (commonly a heater malfunction).

 

Sander, thanks for the input. Fortunately for my wallet, and unfortunately for my automotive education, I sold my 91 Civic LX yesterday for a little less than KBB "good condition" price, to a family who lives in the next town over where emissions testing is not required, so they said they did not care. They listened carefully to the engine, drove it, maybe liked my proof I was the sole owner, and bought it within 20 minutes. Selling the car has been my goal for a few weeks now, before I even learned it would not pass emissions where I was.

Based on your input and other reports, I did not like the readings I was getting for the O2 sensor, so next I was going to replace it. Other things I was considering were checking compression (worn rings could be the problem); replacing the ECT sensor; replacing the o-rings on the fuel injectors; continuing with Chevron Techron for at least four tanks of gas; drive the car daily and try to do higher RPMs under load to help burn off carbon on valves, cyl walls, etc.

I would have been really interested to learn that the problem was the O2 sensor. If it was the problem, then other c. 1990 Civic owners should know the idle and other sounds of the car may not even appear off when the O2 sensor dies.