A plugged catalytic converter will cause a power loss. However, if they are plugged, you should have diagnostic trouble codes stores. For example, P0420, which indicated an issue with the converter. Also, keep in mind fuel pressure can also be dropping and causing the power loss along with other things.
Since you have taken it to a shop, was the computer scanned? If so, were codes found and what were they?
If you want, take a look through these two links. One describes common symptoms of a plugged converter and the other how to test one.
Now, the link indicates to remove the converter. Try just removing an O2 sensor before the converter. Although it will not be perfect, if the converter is plugged, you should notice a difference with the sensor out.
Try that and let me know what you find. By the way, you can't use o2 sensors if it has a straight pipe, so they are being honest. Also, the vehicle's emissions system is required based on federal law, so by law it can't be modified or removed.
Let me know.
Friday, May 22nd, 2020 AT 5:58 PM