Engine light on Toyota 1998

I have a 1998 Toyota Corolla that has at least 1395xx. For the past few weeks, it has been running harder than usual and I most notice it when I am driving at least 30 or 35 mph. Friday, the check engine light came on and it has not gone off since. I took the car to AutoZone where a guy plugged in a sensor and said my emission levels were off and something about repairing or replacing the catilytic converter. Does this make sense? I know it is tied to the exhaust system and I thought the sound was similar to having a hole in the muffler (from previous experience). How soon should I have this fixed…aka how long can I get away with not getting it fixed before it costs me more than just the repairs or replacement? Thanks!

The catalytic converter may be bad; but, something wrong with the engine made it bad. Bring the codes from Auto Zone to here. Somebody can tell you what you can do about them.
Often, when the catalytic converter gets stopped up, it’s because the engine needs a good tune up. This would include spark plugs, maybe spark plug wires, air filter, pcv valve, as a start.
Take it to a mechanic.

not necesarally something in the engine made it bad a cat conv burns off the excess gas from the motor and will clog up on its own but i would try cleaning ur down and up stream o2 sensors with a wire brush first(do it easy)

Without knowing the exact rouble code that is stored or the repair history of the vehicle it is a little difficult to give an accurate suggestion of your trouble but here is my guess.
You have an exhaust leak.
This leak causes the “hole in the muffler noise” and allows fresh air to enter the exhaust system.
This fresh air mixes with the exhaust gas.
The oxygen sensor reads this mixture as lean because of the added fresh air.
The oxygen sensor tells the ECM to add more fuel.
Due to the fresh air the oxygen sensor continues to read lean even though the fuel system is running at maximum enrichment.
This turns the check engine light and causes the motor to run bad because it is running to rich.
So either fix the exhaust leak or provide us with more info.

Getting the exact code like P0123 would be very helpful. Without that we are only guessing what the engine is trying to tell you. Some auto parts stores like Autozone and Advanced Auto Parts will usually read them for free.

The computer knows there’s a problem and trips the CEL light because it compares the reading from the oxygen sensor in front of the catalytic convreter and the one behind it. The signals should be different (the first goes up and down like a sinesoidal wave, the second shows a much more constant and higher level of oxygen). If either signal is outside of its normal characteristics, or if they’re not different, the computer “sees” this and trips the CEL light.

Since you have a known exhaust hole, it’s likely affecting the oxygen sensor readings and tripping the light. Whether the hole is in the converter, a pipe, or a connection is impossible to tell from here. But wherever it is, in needs to be corrected ASAP. An exhaust leak under and just fore of the passenger cabin could allow carbon monoxide into the cabin and that can kill you.