I have an '01 non-turbo S60 standard transmission with 170K on it. Recently I have been getting this message on my dash screen (Emission Service Required). It will stay there for a couple of weeks and then go away for a couple of weeks.

They ran the codes and it found ECM-4201. The mechanic naturally said the cat. needs to be replaced. However, since it is intermittent I am wondering if it's something else. I would hate to spend $1800 replacing the cat. only to have this message continue to come on every 3 weeks.

They recently replaced my cracked EVAP HOSE,VALVE TO CANISTER and the message hasn't returned. I'm not sure if this solved the emission problem or if it is just a coincidence. I need to have my emissions test done next year.

ECM-4201 is stored if the ECM registers that the rear heated oxygen sensor is switching too quickly.

Possible problems are:
1. Air leakage in the exhaust system.
2. One or more damaged injectors.
3. Damaged catalytic converter.

To check for 1 you should have an exhaust pressure regulator which builds up the pressure in the system. The joints in the exhaust between the cylinder head and the rear oxygen sensor are then sprayed with a soap solution to find out if there is a leak. Or, with a cold exhaust and a helper, start the car and stuff a rag in the exhaust pipe and listen for leaks between the manifold and O2 sensor.

To check for 2 you should probably remove them and have them professionally cleaned.

--
My advice is free, so you got what you paid for...

hmmm... The oxygen sensors have never been replaced. Putting in new ones is probably a good idea anyways at 170K and might solve the problem? Thoughts?

If the rear O2 sensor is acting up, it is probably time to replace front and rear sensors. They do have a defined life. The Cat going bad is probably not the issue, that is rare.

Because the Cat pipe contains both O2 sensors, getting a new Cat will solve the problem, but at a considerable expense.

Klaus
--
My advice is free, so you got what you paid for...

I see. I didn't realize buying a new cat. included the front and rear O2 sensors with them?

Thanks for the advice. It looks like $400+ for the sensors on FCPgroton which much cheaper than the new volvo cat. Although I'll probably want to get original volvo over the aftermarket that FCP offers. Cheers

Get OEM from Tasca Volvo:
http://www.tascaparts.com/partlocator/index.cfm?siteid=213668

Most owners will get Bosh replacment O2 sensors.

Klaus
--
My advice is free, so you got what you paid for...

Performed the previous suggestions with no success.

I recently installed 2 new O2 sensors (direct fit aftermarket -denso front and bosch rear). After driving 50miles the check engine light went off and has stayed off for a month now.

Hopefully I am in the clear and don't need a new $1200 cat!

Cheers