Ontario only measures opacity of the exhaust, so whatever else your "catalytic" was doing, the negative effects on the air aren't being measured

Cat conv
 

Caterpillar had catalytic converts inside the muffer. These will plugg quickly during extreme temp changes. They will eventually make you loose power, limit mpd and eventually you wont be able to start unit. Replacing the muffler w.o a catalytic conv is not noticible.

Now In the next teir engines 3 & 4 you have the dpf. You will not be able to remove these as your computer measures the flow across it as well as the temp differentials. People have tries hollowing out the filters so the soot doesnt clog, but the computer pressures and temps have to be withing very tight specs and it can detect these alterations as hollowed out dpf will cause too quick of an air flow.

So you can remove it, they wont be able to tell, but once the computer detects a problem it will derate and not let you go over 10 mph. Also, at that point you will not be able to manually regen system and it will take a licensed dealer computer to reset it.

I had 2 road calls it 2 days that I travelled 200 mi each way to regen the systems and get them to the shop and put a legit dpf.

People have also tried to switch the sensors from the front to the back, but once again the computer detected problem and derated.

Funny how Mitsubishi has a third teir eng. without electronic devices.

the last teir engine (4) , the one that takes cat out of the truck engine business will be thr first with electronics to meet the standards caused from the global warming lie

I have an APU with a three cylinder Mitsubishi engine that is tier three with no extra emission BS. Its rated service life is 25k hours and runs like a charm. I love it.

cat vs mitsubishi
 

Mitsubishi built an entire plant to manufacure CAT engines (Industrial( under the agreement Mit does not sell assemble whole generators in USA.

You can actually watch them put CAT stickers on Mit engines.

I visited the plant in Japan when I was on a job interview... Mit is well ahead of the majors when it comes to tiers !