99 3.0 V6. Check engine light came on. Took to Auto Zone and O'Reilley's Auto parts for free testing. Two different places gave different set of codes. I received P0420 and P0500. Cat system efficiency below threshold Bank 1, and Vehicle Speed Sensor A. The other place said same on Speed Sensor but also said that it the light could be on due to a misfiring plug number 6. Sounds like I have a small hole in exhaust. Could this be a culprit as well. I think I can do the Speed Sensor and an O2 sensor but is it ok to cut off the CAT and go without it. Not sure what to do or where to start.

 

The people at parts stores are not mechanics and offer free Code readings to sell parts, so grain of salt on their diagnose of codes

P0500
In 1999 Rangers(1998-2000) any problem with the "speed sensor" itself will also cause ABS light to come on, is your ABS light on?
These year Rangers SHARE the ABS(anti-lock brakes) speed sensor on the rear axle, is if it was the problem then ABS light would be on
Does speedometer work?
If it does then P0500 is most likely a problem with wiring at computer, or GEM module behind radio, speedometer, cruise and computer all share same speed signal from GEM

P0420
Could be Cat converter is wearing out but this wouldn't cause any running issues at all, it strictly an emissions code, and will turn on CEL
If your engine was sluggish and have lower power at higher RPMs then it could be Cat converter is rusted out inside and is blocking exhaust flow, that would be a problem
Do not change O2 sensor, it it wasn't working then you would NOT get this code, so "don't shoot the messenger"

If you cut out the Cat then this code will be there all the time, since "Cat is not working" which is what the code means
If you remove, or unplug, the rear O2 sensor then you will getP0136 - O2 sensor malfunction Bank 1 sensor 2

P0306 is misfire #6 code
Is this what you got????

 

ABS light is not on and speedometer is working. As far as #6 code, girlfriend took it in had had it read the first time. They said it could be that and showed her the firing order. She also says that at higher RPM in higher gears (4 and 5), it seems sluggish. Had it hammered in 5th and would not gain speed.