Maybe it's a plugged cat?

How many times have you asked yourself that question?

The customer say's it has no power, so you take it for a spin, well, yes or no? If you're well experienced, you will know from the sound it makes, it will sound like a dying moose in the wilderness.
Of course it will certainly have no power, the more you go into the throttle the worse it gets, and we are talking worst case here, so plugged it might even make the  whooshing air sound.
 If you've driven enough of these you learn there traits, most are very similar and it's hard to mistake them for some other problem, but before you get too sure of yourself, you better verify it.

The way I have always done them, is with a quality psi gauge.

This is the actual one I used for quite some time, a Kent - Moore BT - 8205.

So a 3/16" thick wall hose will do, I used the silicone stuff they used for the dpfe sensors on ford egr tubes, now don't laugh about this, buy a cheap telescopic antenna,
and cut out the tube the very upper rod with the ball goes into, this will be your probe tip you will get your pressure reading at the exhaust pipe with, these have a nice rounded tip with a small hole in it,
just make sure the hole you drill in the pipe ahead of the cat is at least 3/16" and the rounded tip of the tube fits the hole in the pipe with out going into it all they way and the rounded portion fits and seals.
Take a small length of hose and attach the probe to the gauge, you will need to hold it to the hole you made in the pipe. Now just start it up and take your measurement about one inch ahead of each cat on the system,
you should not see anymore than 1 to 1-1/2 psi or less at each one, most often you will see 1/2 psi or less on a good system, if its plugged you could see as high as 8-12 psi at idle,
if you raise rpm it should go even higher possibly to beyond 15 psi or more.
Higher than normal back pressure readings means something is restricting the flow of exhaust out the tailpipe.
Though the converter is usually the trouble spot, restrictions can also occur inside mufflers and resonators if a baffle collapses, double-wall exhaust pipes can also collapse internally causing a blockage.

 Once you have done this a few times, it takes a lot less time than going into the o2 sensor opening, especially if there hard to get to, those won't let you look at each cat anyway.
I've seen guys use there labscopes with a pressure sensor, or even look at scan values to determine a plugged cat, and while these work, an awful lot of time is spent with hookups and interpreting results,
my way is much quicker and foolproof than all the others, oh, and those holes you drilled? You can weld can't you?

Another way to check for a back pressure problem is to check intake vacuum at the engine.
Most engines will develop 16 to 22 inches Hg of vacuum at idle. To check vacuum at the intake manifold,
start with the engine off, and disable the EGR by removing or disconnecting its hose or one of its solenoids. Connect a vacuum gauge to a vacuum source on the intake manifold or throttle body.
Start the engine and note the vacuum reading at idle with the transmission in neutral.
If the idle vacuum reading is lower than normal, or it continues to drop while the engine idles,
an exhaust restriction is causing exhaust pressure to build up and backup into the engine.
If you increase engine speed, the vacuum reading will drop slightly,
then stabilize and rise back up to within 2 to 3 inches of the vacuum reading you noted at idle. Any sudden drop of over 10 inches Hg of vacuum will indicate a restriction.

A good vacuum gauge can reveal a lot of different things if you know what to look for, it was actually an important tool in the old days for diagnosis and still can be for finding mechanical problems in
whats basically an air pump. If your unlucky enough to get a no start because of a plugged exhaust, and you wonder if that's the cause do a cranking vacuum test, if vacuum is non existent, now you know,
also if you open the throttle all the way while cranking and it pops very loud repeatedly from the intake is another giveaway, that pressure backs up in the engine and has to go somewhere.

Hold on, hold on, there's yet another way to verify a plugged cat by doing a dynamic compression test, simply remove a spark plug and from your compression tester, remove the shrader valve,
you don't want it to hold pressure at the gauge. Make sure you ground the ignition wire or coil, also unhook the injector wire if equipped, first do a static test and record the reading,
suppose you see a typical 150 psi, this is your static reading. Now go ahead and start it up and look at the running reading, it should be about 50% of the static reading, lets say 75 psi,
now briefly snap the throttle to wot and watch the reading, it should rise to about 80% of the static reading, lets say 125 psi, suppose it goes beyond this reading, lets say maybe 180 or more psi,
that indicates a plugged exhaust, but lets just do another cylinder just to be sure, and see if we get the same result, if so your exhaust is plugged for whatever reason.

After giving some thought to this, I felt I should include looking at scan tool values to quick check for restriction, but certainly not for verification, the one value in particular would be the map sensor,

using the chart above will give you an idea what you would look at, during engine idle, map should be around between 30-40 kpa which would indicate a vacuum reading between,
18-19 inches, map volts would be about 1.1-1.3 that's the number you would see on your scan tool if everything is normal, but if you see a volt reading that's higher, maybe 2.2 or higher,

which would indicate a lower vacuum reading, higher manifold pressure, and a possible restriction,  if you open the throttle slowly bringing rpm up, and volts increase quickly to beyond 3.8-4.0 or more,
now your seeing a restriction, your building more pressure since it has nowhere to go. Still, use a mechanical method of verification, always. Possibly the maf sensor would help
here too, air flow readings should be much lower than normal, it might be worth looking at if you have a restriction just for future reference.