I am keeping two Volvo cars on the road for one year so far: a 94 -Mod 850 and a 01-Mod V70 both wagons. I have done struts, timing belt thermostat water pump, motor mounts. In the past I have done rebuilds of head valve guides seals and had crank turned and honed cylinders to 0.025 over replaced bearings oil pumps, rings and pistons so you could say I have some experience with limited makes of cars.
These days on my 01 V70, The car has low performance, and I am trying to clear a code for "Catalytic efficiency" O2 sensor upstream vs. down (both located in the CAT) and have ruled out most possibilities like intake fuel air leak exhaust leak etc per my Haynes repair manual and "all data" so now I am not sure how to test for a clogged CAT verses the bad O2 sensors. What I observe on my scanner so far is that one of the O2 sensors is displaying negative numbers and low values compared to the other. I am tempted to just change it before checking for a CAT restriction because both o2 sensors will get no flow if the CAT is removed.
I'm not sure posing this question here is the proper place, but any discussion or guidance for this type of question would be appreciated

 

Welcome to the forum. The best place for a question like this would be the V70 forum. I would recommend posting it there.

 

Welcome to the forum. Moving to V70 forum.

 

1. You can check the pressure differential between the 02 sensor ports. Difference should not be more than 1.5 psi @ 2500 RPM

2. There should never be a negative voltage, and there should not be much difference between the two when the system is working well. A rich mixture will create a higher voltage than lean. A good read is .45v.

If everything is working right the converter doesn't work too hard.
Ed

 

Thanks Ed7,
Great info on checking the delta pressure. I am tring to get the thread adapter ready for my 1/4-NPT male pressure gauge so I can do the test.
I measure the male thread of the O2 sensor to be 17mm x1.5 pitch and have a brass fitting that is pretty close to size at 3/4-16 with some teflon tape.
I will change the front o2 sensor since it is going negative on the scan and then do the pressure test since I don't have any eggish smells coming from the Cat indicationg a clog right now.

 

Old spark plugs can be modified to fit the o2 holes. Screw the 1/4" npt fittings into those after you knock the insulator out. You'll need a manometer or a sensitive low pressure gauge to read.
Get some clear tygon tubing and make a U-tube manometer. Dwyer Instruments - Primer On How Our Products Work using water to read the pressure.
This is just a U shape tube with a measuring tape in the middle to read the difference.
One PSI is equal to 27.7 inches of water column.
This equals about 14" displacement on either side of the tube. 14" up and 14" down. This means you'll need about 40" of clear tubing (any size) plus length to get to the pipe.
and it needs to stand straight up and down.
Ed

 

I was thinking the o2 sensor thread looked like a spark plug thread, thanks now I don’t need to look it up. :-)
The spark plug has a nice thick wall for cutting an NPT thread in the center. I like the home made manometer info but in my case … I borrowed a $$ digital pressure gage from work that measures up to 100 psi in increments of .oo1 psi I was planning to take two readings, each at 2500 rpm at each o2 thread position (keeping the other o2 sensor in place) and subtract for the delta pressure?
I hope the readings don’t jump around a lot in the 0.1 PSI column anyway.
I am not sure of my method because, a simultaneous reading of differential pressure from both o2 fittings would assure that both get the same RPM pressure affect as compared to two separate test readings.
I can get a Dwyer manometer from work instead and make two fittings with tubing?

 

With the water manometer there's no math involved. With the manometer connected to each port you get the differential only as the engine speed changes. You're only looking at 7 to 14 inches of water column(1/4-1/2psi), that's 7"up and 7"down = 14" total displaced.
Your engine won't like the o2 sensor being out and will want to dump more fuel into the engine. Test just long enough to get a read.
Ed