Question about 1996 GMC Yukon

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02 sensors not ready

: I replaced all 4 O2 sensors on my 1996 GMC 5.7L Yukon. My scan tool shows "Sensors not ready to communicate or available" for #2 &
#4 ( downstream. #1 & #3 are fine and reading.)

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  • jdarling May 14, 2009

    The code is P0420 but when you run a sensor check on the scanner that is the message I get

  • Dave  C
    Dave C May 11, 2010

    You'll simply need to drive it further. There is a drive cycle to reset the readiness monitors. I will see if I can dig it up.

  • mike loshbough
    mike loshbough May 11, 2010

    what are the code #s

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The code P0420 indicates that the catalytic converter is not functioning efficiently. This could be because the cat is damaged or the engine is not running very well and the cat cannot keep up with the emissions. You should ensure that the spark plugs and ignition components are in top shape before replacing anything else. If a compression check reveals a low cylinder, you will not be able to prevent this code as even a new cat cannot keep up with a dead miss. If the state of tune and the compression is good, then you'll likely need a cat.

I've posted the drive cycle just so you know what to do when you have resolved the code.

  1. Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature at startup.
    Do not leave the key in prior to the cold start or the heated oxygen sensor diagnostic may not run.
  2. Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire, and - if closed loop is achieved- Fuel Trim.
  3. Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph (88km/hr) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
  4. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
  5. Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 20 mph (32km/hr). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
  6. Accelerate. Accelerate at 3/4 throttle until 55-60mph (88-96 km/hr). This will perform the same diagnostics as in step 3.
  7. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
  8. Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

Posted on May 14, 2009

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This code means your catalytic converter is below efficiency status .the o2 sensors will not fix this problem ,the ceramic in the converter is broke down ,so the converter it self is the problem.So many people think that it is a bad o2 but it is the converter!

Posted on May 14, 2009

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