The lambda light came on in my 98 S70 non turbo (340,000Kms on the clock). My mechanic found the following:

MIL light on,
FRT 436, REAR L025
Compensation P0133

Okay, so the rear O2 sensor is done and needs replacement. We do that and reset. A day later the lambda light comes back on. Here is what the mechanic found:

MIL light on,
FET 436 REAR H025
Compensation P0133
FET 443 TWC Efficiency
P0422

Okay, so the front 02 sensor now needs replacement. We do that and reset.
A day later the lambda light comes back on. Now he tells me I need a catalytic converter replacement! What the heck? I should mention that my mechanic installed Bosch O2 sensors to save me money but I've heard these replacements are iffy at best as a replacement for the Volvo OEM's.

I thought a CAT only failed if it rotted out or became plugged up? Structurally the CAT looks okay.

So, what should I do? I cannot obtain a used CAT because all the auto salvage yards here send these things to a recycler for the platinum. They won't resell used CAT's. I hate to spend $1300 on a CAT replacement if that is not the problem. I only need to pass my E-test this year and then I'll have a different vehicle in 24 months. What is the resolution?

I found this thread in the UK:

http://www.volvoforums.org.uk/showthread.php?t=141000

While it does not resolve the issue, it will give you a few ideas as to what is going wrong. I am having the same problem with a 2001 Audi, the dealer has no clue and wants to throw a new cat on it. I think that there is a vacuum leak somewhere messing with the fuel mixture.

The rear O2 sensor was still good, it was reporting data just as it should. Perhaps a good 'hot' run after looking for leaks. Don't forget to look for a leak in the exhaust before the cat.
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My name is Klaus and I am a V♂lv♂holic

Klaus:

Thanks for the reply. This car is driven mostly highway miles, so I don't suspect a fouled CAT. I'm just frustrated that after 2 O2 sensors the problem is still there. I hate taking the vehicle into the local dealer, however I think this is my only option as I believe my independent mechanic doesn't have a definitive answer to this problem. A related problem might be my infrequent cruise control on this vehicle. Some times the cruise works, other days nothing.

I'll likely take the car into the local dealer next week for diagnosis. If I obtain a solution to this, I'll let you know.

Your 1998 cruis control is a stand alone system with its own vacuum pump (under the battery tray). Usually the fault there is in the switch above the brake pedal. Or, like my 1998, the vacuum pump finally gave up, not enough vacuum to keep the car at speed.

Please, do look for vacuum leaks. A porus elbow can cause the engine to run rich and mess with the O2 sensors. Start at the vacuum tree, under the plastic cover of the throttle.

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My name is Klaus and I am a V♂lv♂holic

Klaus:

Thank you very much for your help with my issue. I took the vehicle into the local Volvo dealer today for diagnosis and they confirmed my worst suspicion. The CAT is done. No vacuum leaks anywhere and the O2 sensors are within spec. So now I have to find a CAT somewhere reasonable in price. I think I'll start with IPD first.

BTW, the labor rate at my local Volvo dealer is now $180 per hour. Ouch!

Cheers.

You can search craiglist and find some one who is selling the car for parts.

I believe tasca price for the catalytic is ~ 800. I am ruuning a

catalytic from a V70 in a 850, these are interchangeable!!!