My yukon is running rough. i just replaced the front O2 Sensor (in front of the catalytic converter), I didn't replace the back sensor. The yukon ran fine for about a week then the check engine light came on and it is doing the same thing again. When I step of the accelerator pedal, black smoke comes from the exhaust pipe. Should I change both sensors now because of this issue or is there something else going on? Thanks

why did you had the Cat change? cat converter it the heart for emission control
you had to change all the O2 sensors
Scan the car and see if all the o2 sensor are working when the car is operating temperature
after this you can see what is happening with the car and emission

i have replaced an O2 sensor and had similar failure after 1500 miles.
I removed it, chkd it, reinstalled it and it ran fine.
P0133 is related to slow response of O2 sensor. P1134 is mfg specific so google that for ur car.

Did you replace the O2 sensor after the catalytic converter? This code is set by the rear O2 sensor. It usually indicates early failure or restricted catalytic converter. May want to have an exhaust back pressure test performed.

This sounds like the mass airflow sensor.

1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Possible Causes: Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
Diagnostic Help:
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101", if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.

if it comes back, yes you will need. without pre o2 sensor your car wont idle, it will use lots of fuel and you will face drivability problems if it is post o2 sensor, it is there to check if catalytic converter is ok or not(it won caue drivability problems but may consume little more fuel to run rich and to save catalytic converter). if sensor is broken you need to replace it. if you have error for post o2 sensor but it is ok, this means your catalytic converter is going bad.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

You will likely not notice any drivability problems, although there may be symptoms.

Causes: A code P0420 may mean that one or more of the following has happened:
Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings


Some suggested steps for troubleshooting a P0420 error code include:
Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

Hope this helps (remember to rate and comment this answer).

ok first of all did you replace the bulbs for the reverse lights ?
the sensors that you replaced are they the o2 sensors or ??
and if you havent replaced the o2 sensors i would start with the one behind you catalytic converter because its most likely going to be that o2 senso or its going to mean you have a bad catalytic converter and its not doing its job right anymore.

Basically, your on board computer is telling you your catalytic converter is not working properly. As you know, you have a sensor in front and in back of the converter. The computer compares the waveforms from each sensor. If the sensor waveform before and after the cat are the same, then the cat is not working. You should have an analysis of the waveforms at each sensor analyzed at a shop and, if they are the same, the dealer should replace the cat if it is still under warranty. Most common would be the O2 sensor after the Catalytic converter has failed and will need to be replaced. It takes about 15 min to replace and any muffler shop can replace the O2 sensor for you.

Good luck

You should have an O2 sensor on each side of the exhaust down pipe, (diver's and passenger side) have one just before the catalytic converter and one after the catalytic converter. That's all the four O2 sensors the Yukon has.