I have high NOX from my emissions test on my 940 91 Turbo (not Mercedes; Volov, but the principle is the same) even though I recently replaced the CAT. I called the CAT OEM and they said "Check the O2 sensor"

I had recently replaced my O2 sensor with a Bosch Ford Mustang 92 GT O2 sensor, which everyone on the Volvo Forum says you can do, but the OEM says you can not.
So I scoped it. I found an example waveform for a Volvo 940 Turbo below on the Internet.

My scope output was pretty much the same, so I am confused as to whether the O2 sensor is at fault.

Got any ideas?

the only way to find out is to plug in the actual Volvo one and compare the two sweeps.
I just received the Volvo sensor in the mail.
I get what you say about response time. When I induce a vacuum leak or change my throttle rpms from idle to 3 K I see a response on the scope, but it takes some time for the waveforms to show up on the scope.
So tonite I will plug in the new O2 sensor and see if the scope shows a faster response in reply to a a vacuum leak or if I change my throttle rpms from idle to 3 grand.
At least the info I found on the Internet gives me some idea on how to set my Time Base and Trigger.
I am also going to do my AMASS and Injectors...too bad I dont have the PicoTech hardware that lets you use a Laptop as a scope..

A faulity O2 sensor is seldom the cause of high NOx, but HC or CO can point to the O2 sensor.

If your car has an EGR system, do a complete functional test. If not figure out how to retard the timing.

Look at the reduction in NOx due to retarding the timing in the following thread and the thread it refers to.

http://www.peachparts.com/shopforum/showthread.php?p=833484#post833484

I could offer a more comprehensive diagnosis if you would post the complete emission test results - all gases including O2 and the type of test/test conditons - ASM? I/M 240?...

Duke

HC, CO passed
NO X failed at 2325 ppm, 1200 = passing

I did not think the O2 sensor was bad...I still think its the Catalytic...

Timing is computer controlled by the ECU so I dont think I can retard it that easily

No EGR on this car

Other causes for high NOX are:
1 Low fuel pressure
2 Dirt Cylinder Head with carbon deposits
3 Hi Op Temperature

Asking a cat to tackle 2300ppm is asking alot. I don't think I'd look at the cat. Using a lower grade fuel than the car calls for can increase nox, but again not that much. The 3rd cause you posted may be the key. The engine may not be geting rid of the heat.I am still waiting for the chance to prove this myself, but several months back a guy posted that his 107 SL was running 1200ppm. He replaced the radiator for different reason and nox went to way below passing.


Peter

__________________
Auto Zentral Ltd.

Given that my electric fan was cycling, which placed the temperature in the 100-105C range, I challenge anyone to explain how NOx was reduced 90 percent from the previous test. The answer is in the theads I referred too, but if you don't want to believe it,...

Nothing more I can do in terms of diagnosis on this one - not enough data - too may myths!

Duke

by the car manufacturer. the radiator has been replaced. I was using 89 because i rarely pushed the car, drove it at 55 like an old lady, even though it was Turbo and Volvo specifies 92 octane on the Gas Cap.

Did I goof? By using the 89 octane? I figured since I was driving Like an old lady, I didnt run the risk of detonation and didnt need that hi Octane. I may have been wrong.

The radiator was replaced 5,000 miles ago. I'm going to change the Thermostat this afternoon. And get some new plugs...

Finally the NOx figure. I might not have got the units right but I did get the ratio right; it was 2300 vice 1199 passing...

The car passed fine two years ago and has been very well maintained...