[An updated version of the following material will likely appear in the next 700-900 FAQ -until then I'm posting this here for the archives]

If the catalytic converter on your 200/700/900 needs replacement or you need to remove the cat for some other reason (such as replacing a leaking gasket or replacing the exhaust pipe or checking for a plugged cat), you will likely have difficulty cracking open the flange that joins the cat to the exhaust pipe.

Depending on the model/year/market, this flange is commonly held together with three bolts in one of these ways:
- Bolts (hex cap screws) going through the exhaust flange and thread into the cat flange. The heads of the bolts may be tack welded to the exhaust flange to prevent tampering with the emissions system. Between the flanges is a thick metallic gasket.
- Press bolts (partly threaded capped studs) pressed into the exhaust flange section. The threaded portion slides through the cat flange which is then fastened tight with nuts. Between the flanges is a do-nut seal that fits between the tapered pipe openings.

Typically the bolts are rusted to the point that the heads/nuts are useless and the threads totally frozen. To make matters worse, the bolts are high strength steel and the heads/nuts may be tack welded in place.

Here's some tips that may help.

o The bolt heads (or stud nuts) have to be removed. You need to cut or grind them flush at the flange in order to get past any tack welds and the rust. You'll start to see the outline of the bolt shank when you're level with the face. As for the weapon of choice:
- A hacksaw is probably hopeless. Even with a stock of replacement blades you don't have much clearance to work and these bolts are hardened. However, a sabre saw with a relatively short metal cutting blade may work okay.
- An oxy-acetylene torch may allow you to heat the bolts up enough to chisel them off. I'm guessing this is the muffler shop approach.
- An angle grinder or air powered cut-off tool with a coarse abrasive wheel works well. With limited clearance around the top bolts, a small wheel (like 3") is best. It would be difficult to use a 5" wheel. I used a 4-1/2" wheel, but I had to force the exhaust away from the chassis to give me adequate room.
- A high speed moto-tool with a suitable (diamond) tip might prove to be the simplest of all.
- Forget about using a grinding wheel chucked in a drill or a flat file for anything but finishing touches.

o The flange should now pry apart fairly easily just by rocking the cat section around a bit -a flat wrecking bar, large slot screw driver or cold chisel may also help here. To remove the cat you'll either need to separate the pipe joint between the cat and the front muffler or, unless you've got an over-axle pipe, do as I did and just pry off all the rubber exhaust mounts and pull the whole exhaust system out from the back. Chisel off the old flange gasket (if it's really in good shape you can just leave it and add a new second gasket).

o If your exhaust flange used press bolts, you'll need to remove the old ones one of the following ways:
- Volvo has a special tool (999 2862) for this, but you won't need it.
- A portable vise (lifted off the work bench if needed) can be used as a press. A chunky c-clamp may also do the job. Use a suitably sized socket as the back spacer.
- Heat the press bolts (preferably until red -a good job for oxy-acetylene). Drive the studs out with a hammer or you may be able to just wrench them out.
- Or you could cut the studs flush and drill them out. As you can easily use bolts instead of pressing in new studs, you don't need to be overly precise. If you really want to replace the studs, they're available from Volvo as p/n 1276215-9.

o If your cat has the threaded flange and you're going to be re-using the cat section, you need to get the remaining threaded studs out of the cat flange. You should now have about 1/2" of threaded bolt sticking out.
- Break the thread bond by heating the stud (preferably until red -a good job for oxy-acetylene) then use large vise-grip pliers to twist out the stud. You could also break the bond by repeated applications of penetrating oil and hammering, but don't get your hopes up.
- Another way is to cut the studs flush at the flange and simply drill the remainder out. As you don't need to re-thread the holes you don't need to be overly precise.

o For the re-installation you'll want to use new bolts going right through both flanges with a lock washer and bolt on the end.
- Use high strength bolts for this application, either something like an 8mmx30mm grade 8.8 metric or a 5/16"x1-1/2" grade 5 SAE cap screw. Use anti-seize on the threads if you've got it.
- If you're re-using the old cat or had to press out studs from the exhaust flange, you need to drill out those holes to accommodate the new bolts. Making the holes slightly oversize makes re-alignment easier.
- To help re-seal the flange, apply gasket cement, RTV sealant or muffler cement. A non-hardening variety will make it easier for the next poor sod to get it apart. In any case, the sealant needs to be rated for high heat applications (like Permatex copper, super blue or blue).

o Although cat converters are supposedly designed to last the life of the vehicle, it is probably not abnormal to see a cat simply wear out after 10 years or 100K miles of service. Anything less than that may be cause for concern. If the catalytic converter fails (plugs up or breaks up) you should consider if one or more of the following has contributed to an early demise:
- Use of leaded gas or "snake oil" additives.
- Very rough engine running (extended periods of misfiring).
- Exhaust gases too hot (excessive lean mix or extended periods running on a dynamometer).
- Excessive vibration (bottoming out on medians or extended running on badly pot-holed or rough gravel roads).
- An inferior quality replacement cat was installed.

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Cautionary Notes:

When raising the car, do not leave hydraulic jacks or vehicle service jacks as final supports -use proper jackstands or ramps. Work only on level, hard ground. Before crawling under for the first time, give the car a good rock side-to-side and front-to-back to ensure completely solid footing.

When working with open flame or sparks under the car, wear suitable protective clothing, keep long hair safe, have an extinguisher within easy reach, keep the hood open, make sure there are no gas leaks and work in open air.

Use proper eye and face protection. Especially when working with abrasive wheels, if the wheel should ever chip, crack and break at 10,000 rpm your dimestore goggles or $600 esigner eyeglasses likely won't be of much help. Keep yourself out of the plane of wheel rotation.