Yes, monitoring O2 sensor voltage can reveal quite a bit. Since a failing sensor can still have "correct" average voltage but be sluggish in operation, an oscilloscope is a better choice than a voltmeter to see what's happening. However, an O2 sensor that checks bad on a voltmeter is bad. When working correctly (once things are sufficiently warm), the voltage fluctuates around 0.45 volts or so. You can make the mixture leaner by pulling a vacuum hose, or richer by temporarily partially blocking the air intake. The O2 sensor voltage should change very rapidly after you do either, but in opposite directions. Rather than explain the theory and what to expect, I found a Web page that says it well:

http://www.aa1car.com/library/o2sensor.htm

This is good info for generic O2 sensor.

However, the 2000+ subarus have WIDEBAND sensor also known as A/F ratio sensor. It doesn't switch around 0.45V like the traditional sensor but rather keeps slightly wavy almost streigt line on oscyloscope or OBD2 interface, its near 4V. Hard to interpret if the sensor is borderline.

 

Believe me, I went through the whole learning process when there was little info available on these new sensors.

This is good info for generic O2 sensor.

However, the 2000+ subarus have WIDEBAND sensor also known as A/F ratio sensor. It doesn't switch around 0.45V like the traditional sensor but rather keeps slightly wavy almost streigt line on oscyloscope or OBD2 interface, its near 4V. Hard to interpret if the sensor is borderline.

Sorry, I didn't realize that the last year Subaru used narrow-band sensors was model year 1999. Wideband sensors don't generate voltage, unlike the earlier design; they rapidly change in resistance (indeed responding to A/F ratio directly), and that resistance change is used to cause a varying voltage.

 

Leaning and enriching the mixture to check the sensor's response as I suggested previously should still work, but the result of that would probably be most easily observed on 2000+ models via a scanner. I do agree that a borderline sensor might be harder to detect.

Sorry, I didn't realize that the last year Subaru used narrow-band sensors was model year 1999. Wideband sensors don't generate voltage, unlike the earlier design; they rapidly change in resistance (indeed responding to A/F ratio directly), and that resistance change is used to cause a varying voltage.

 

Leaning and enriching the mixture to check the sensor's response as I suggested previously should still work, but the result of that would probably be most easily observed on 2000+ models via a scanner. I do agree that a borderline sensor might be harder to detect.

 

Can the new style sensors be safely measured with an ohmmeter? Would that be helpful or can they still be 'lazy'?

 

Carl

One of my friends who is now with our IT department used to be the guy the dealers called when they could not isolate a problem. He says the only accurate way is to have them on a scanner and watch them operate.

can i use any obd2 scanner or does it half to be a subi scanner

can i use any obd2 scanner or does it half to be a subi scanner

 

there are 3-4 OBDII protocols so it is POSSIBLE to get a scanner that won't read a soob or get one with limited functions that won't clear a CEL I guess. The soob protocol is ISO something IIRC. It DOES NOT have to be a Subaru/FHI tool. Innova si one that gets mentioned a lot as well as some that are laptop based software.

 

Carl

I think you have to make sure it is compatible. Some of the guys have laptop software, perhaps they will chime in.