While your car's emissions and transmission systems may seem unconnected, the fact is that modern cars are so intricately networked that one can easily affect the other. Diagnosis can be difficult, but knowing how your car works is a good start.
Blocked Converter Detection
All cars produced after 1996 have at least two oxygen sensors, one before and one after the converter. Blocked catalytic converter passages will force exhaust heat to exit through the remaining passages, trapping it inside the converter and causing its internal temperature to skyrocket. The oxygen sensor detects this temperature difference and notifies the computer.
Upon detecting severe converter blockage, the computer will automatically reduce timing and the air/fuel ratio to prevent engine damage and complete converter meltdown. This "limp home" mode can also include changes to the transmission programming to reduce load on the engine. These changes can include locking out the overdrive, changing shift firmness, reducing shift RPM and changing the torque converter lock-up point.
A blocked converter can reach over 649 degrees Celsius internally, and eventually the torque converter case can begin to glow red hot. These kinds of temperatures can easily boil transmission fluid if the converter is too close to the transmission or transmission cooler lines. This can manifest as torque converter slippage, clutch slippage, excess RPM and possible outright transmission failure.