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Yesterday I ventured on replacing a failed catalytic converter on my own on an 02 Nissan Pathfinder and it was smooth going until a couple of the bolts broke when removing the old cat.
This would have been a straight-forward operation, but of course, now it's more complicated.
#1 is broken flush, and #2 is broken but still protruding.
A couple of questions I have:
First of all, get some penetrating oil. I like PB Blaster but there are other brands. WD-40 is not a good choice. Liberally spray both sides and give it some time to penetrate. Overnight is a reasonable time.
Then for the ones that are still sticking out, use an external stud extractor or a pair of good quality locking pliers to try to remove them. It can take some time to work it back and forth. Sometimes heat from a propane torch can help as well.
For the ones are are snapped off, you'll need to drill those out. Usually once you drill a reasonably large hole in the stud without damaging the threads on the assembly, the stud will loosen up. If not, try the penetrating oil again, and use an internal screw extractor to get it out. Again heat can often help as well.
Get a thread chaser of the correct size and pitch and clean out the threads before you re-assemble with new studs.
If all of those fail and you still can't get them out or if there is not enough room to work, you may need to remove the entire header assembly. But that's a last resort as it's going to be a lot of work.
While it may work what is posted in the other answer, your best bet is to cut the protruding one flat, then drill them out, enlarge the hole, and put a through bolt into them. With cats and exhaust, when they get like this, there's just about no good way to remove or fix studs which get into this condition. Probably your only other safe bet is completely replacement of the part.
Getting those out in that confined space could be difficult. If all else fails, I would be tempted to just drill them out, then put a couple of nuts on the back of the flange and if necessary some longer bolts. There looks to be decent amount of space for some nuts on the back of the flange, so as long as there is a flat surface, it should work.
Looks like there's enough room to work there, so grind off till flat, use a centre punch to get your first pilot hole through (perpendicular, please) then gradually enlarge until you can re-tap the thread appropriately. Or, as already suggested, use an Easy-out - left hand threaded removal tool, when the hole is looking nearly there. The heat from the drilling may well have eased the problem by then.
If that goes pear shaped (the hole!) then go slightly larger, to enable a bolt to go straight throug, with a nut on the other end. It looks like they may be studs rather than bolts, but that's beside the point.
Before re-assembling, clear out the threads for all concerned, and use copperslip or suchlike when re-assembling, making the job easier for the next time.
'Twere me, I'd maybe heat up round the thread of the protruding stud, and try a mole wrench on it - but only for a couple of minutes. Then plan A.
