As above, or to quote a piece of text written by Tom Denton in his rather excellent book: Advanced Automotive Fault Diagnosis.

"Misfire monitor -
When an engine endures a period of misfire, at
best tailpipe emissions will increase and at worst catalyst damage and even destruction can occur.
When misfire occurs, the unburned fuel and air is
discharged direct to the exhaust system where it
passes directly through the catalyst.
Subsequent normal combustion events can combust this air/fuel charge in something akin to a bellows effect, which causes catalyst temperatures
to rise considerably.
Catalyst damage failure thresholds are package specific but are in the region of 1000°C. The catalyst itself is a very expensive service item whether replaced by the customer or the manufacturer under warranty.
The misfire monitor is responsible for determining
when misfire has occurred, calculating the
rate of engine misfire and then initiating some
kind of protective action in order to prevent catalyst damage.
The misfire monitor is in operation continuously within a ‘calibrateable’ engine speed/load window defined by the legislation.
The USA requires misfire monitoring throughout the revs range but European legislation requires monitoring only up to 4500 rev/min.
The crankshaft sensor generates a signal as the
wheel rotates and the microprocessor processes this signal to determine the angular acceleration
of the crankshaft produced by each engine cylinder
when a firing event occurs. When a misfire
occurs the crankshaft decelerates and a cam position sensor identifies the cylinder that misfired.
Processing of the signal from the crank position
sensor is not straightforward. A considerable
amount of post-processing takes place to filter the signal and disable monitoring in unfavourable
conditions. The misfire monitor must learn and
cater for the differences in manufacturing tolerances of the crankshaft wheel and so has a specific sub-algorithm for learning these differences and allowing for them when calculating the angular acceleration of the crankshaft.
These correction factors are calculated during
deceleration, with the injectors switched off. They should be re-learned following driveline component changes such as flywheel, torque converter, crankshaft sensor, etc.
The misfire monitor must be able to detect
two types of misfire:
● Type A misfire;
● Type B misfire.
A type A misfire is defined as that rate of misfire, which causes catalyst damage. When this occurs the MI will flash at a rate of 1 Hz and is allowed to stop flashing should the misfire disappear. The MI will stay on steady state should the misfire re-occur on a subsequent drive and the engine operating conditions are ‘similar’ i.e. engine speed is within 375 rev/min, engine load is within 20%, and the engine’s warm-up status is the same as that under which the malfunction was first detected (and no
new malfunctions have been detected). The rate of misfire that will cause catalyst damage varies as a function of engine speed and load.
Misfire rates in the region of 45% are required to
damage a catalyst at neutral idle whilst at 80%
engine load and 4000 rev/min misfire rates in the
region of only 5% are needed.
A type B misfire is defined as that rate of misfire that will cause the tailpipe emissions to exceed legislated levels. This varies from vehicle to vehicle and is dependent upon catalyst package. MI operation is the same as for standard MI DTCs."