Volkswagen Repair: 2000 Jetta Cylinder & Sensor Errors, oxygen sensors, cylinder sensor

Question
Hello. My 2000 Jetta has been losing power and the engine also began smoking. The error codes are PO 300, PO 304, PO 302, PO 301 and PO 137. The first four errors state that the cylinders are misfiring and the last suggests low volts for sensor circuit. Should I change the oxygen sensors and how do I know if the cadalytic converter is good or bad? What other suggestions do you have? Thanks for your help.

Answer
The best way to check the sensor is with the oscilloscope. Most people don't have a scope. The next best way is with a factory scan tool. The dtc is P0137 O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2) –   Check the Oxygen Sensor (O2S) Behind Three Way Catalytic Converter (TWC) -G130-.  
Short to ground <-0.15 V
Most of the time the sensor is bad this can also be the wiring. This is the rear sensor and not the front. The purpose of the senor is to check the efecienticy of the catalyst by seeing the difference from the front o2 sensor. The battery being discharged can also cause this fault. You did not include the type of engine and that is all the difference that my answer will have. Vin # will be best. All cylinders misfiring may be the secondary ignition coil or a Air fuel problem. If this is a turbo engine and the smoke is grey and smells like burnt oil then most likely the turbo is cracked. If it smells like rotten eggs and is grayish black then its because of the misfires. The smoke and the misfires are probably related tell me what color smell and be a lot more descriptive on how your loosing power and the vin. Change the rear O and retest. This will not fix the missfires but almost all the time fix that fault. every missfire is killing the cat. its unknown if the cat is ok till you fix the o2sensor fault and let the ecm check it. The factory scan tool is the best way to check the catalyst.