Hello all;
I am back again and trying to figure out my next move. I have had the annoying problem of my CEL illuminating and I can't seem to turn it off for good or at least to get it thru PA state inspection.

Here is the latest. Driving in to work yesterday morning and the CAT converter broke away from its rear connection pipe. Embarassing noise to say the least. Made it to work and dropped it off at my local Indy shop. He replaced the CAT with a new one and erased all codes which turned off the CEL. I was hoping that replacing the CAT would take care of this issue but low and behold after only 35 miles of driving the CEL is back on. I pulled two codes this morning in the parking lot. (P0133 and P0140)

I have already replaced both the front and aft O2 sensors at least once within the last year. Replaced with Bosch. I drove the car all of this past winter with the CEL on so can anyone tell me can a bad CAT burn out a new O2 sensor?
Also I have not yet attacked any of the vacuum lines as might have been suggested to me previously.
Tired of throwing money at this car just to get the CEL turned off. Any suggestions would be much appreciated as I have an August 2015 state inspection due and they won't touch the car with the CEL turned on.

1998 Volvo S70 (Base) 147k miles

The 133 is a slow responding front sensor. Please look for vacuum line breaks, that is a cheap option.

The 140 is a non-responding rear O2 sensor. Check the wires. Perhaps the exhaust guy didn't seat the connector properly or broke a wire when threading the sensor to the new converter.

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My back feels better when I sit in a Volvo seat

Klaus, can you please explain to me how a broken or leaking vacuum line can cause a P0133 code to trip?

Any air coming into the system after the MAF/AMM will cause a rich condition at the spark plugs. Initially, the computer sees a lean burn at the front O2 sensor and adjusts the fuel to increase to make up for it. Often associated with a long term lean/rich error, which may or may not trigger a fault.

The rear O2 sensor is just a monitor to make sure the front O2 sensor is doing its job. Some scanners can follow the O2 sensors and graph the electrical flow, but a P0140 is a wiring or plug problem. If the mechanic didn't unplug the sensors before removing them, the wires will get twisted badly and stretched. If the plug end was removed, there may be dirt or grease in the area causing a bad contact.

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My back feels better when I sit in a Volvo seat

Klaus,
I plan to do what you said and first attack all of my vacuum lines.
I plan to also replace all of the elbows and that one tree.

I have another rleated question here. Some other Volvo expert
on another board asked me if I had replaced my CAT converter with an O.E. Volvo one. I said no. I replaced the old CAT wiht an aftermarket "Walker".
This guy seemed to think that it has to be an O.E. CAT or I will continue to have the CEL on with codes. is that true in your opinion?

California is the only state that requires OEM cat converters. While 'aftermarket' Cats are not as good as OEM, they still work quite well and will last for years.

The most effective part of the system is the O2 sensors. Due to Volvo's stricter than required emmissions policy, the O2 sensors need to react very quickly and some of the aftermarket do not, or they loose capacity in 10-20K miles.

Check the exhaust system for leaks before the cat also.

No need to replace the vacuum 'tree' unless it is cracked and/or leaking.

Your engine has a hidden vacuum line close to the #1 cyl intake. It is notorious for cracking due to heat. Very hard to see, so just feel with fingers, any roughness will probably determine if it need replacement.

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My back feels better when I sit in a Volvo seat

Klaus,
Thanks for your response.
If I wanted to have a professional check my exhaust system for leaks before the CAT, who would you take it to other than a dealer?
Would a muffler shop do?

The same guy that welds mufflers.
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My back feels better when I sit in a Volvo seat

http://www.matthewsvolvosite.com/forums/viewtopic.php?t=33083

(Read all the entries in the above thread)

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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the ignition - now back to the brakes again)

Thanks Klaus.
Usually the front sensor doesn't fail they say.
Is there a way to check the functionality of the rear O2 sensor?
(i.e. meter or speial equipment needed)

I believe the O2 sensor preheater is powered through a fuse. You might check it to see if the old sensor or wiring shorted and blew it.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the ignition - now back to the brakes again)

Chris,
I don;t think there is any fse for what you stated.

See the followijg link: http://www.matthewsvolvosite.com/fuse-box-location-volvo-850-s70-v70-xc70-c70.html

It looks like it shares a fuse with other system(s):

http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=50864

So if it WAS a blown fuse, you would probably be lacking other systems also.

Curious - do you still have the original oxygen sensors laying around? If so, can you read the brand name? Some of these cars,generally the non-turbos I think, use Denso or maybe even NTK oxygen sensors. There have been many web posts over the years that suggest if you don't replace them with an exact plug-and-play OE part that you are flirting with incompatibility/failure - that means no "universal fit" or "adapted" parts. I just can't find anywhere on the web that spells out who the OE manufacturer is for your particular sensors.

Also, did you replace both sensors at the same time? Any chance you have them mixed up?
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the ignition - now back to the brakes again)

Heh Chris,
No I do not have the original sensors. They got tossed awhile ago.
I bought a NGK when i replaced the front one and the rear was Bosch.
I always bought the ones with the correct attached wire harnesses. Never universal fit.
They were both replaced at the same time but last year one Indy mechanic told me that the rear one was shot after only 6 months or so due to the bad CAT converter. They replaced the rear one with another Bosch.
I find it hard to believe that the car is smart enough to know what brand O2 sensor you put in the car. :)

Chis,
By "pre-heater" do you mean the front sensor? (i.e. before the CAT)
I believe the fuse box is under the hood, right?