Engine light - Code P0420

Discussion in 'Tech Info' started by cuevrojamez, Dec 13, 2016.

  1. cuevrojamez

    cuevrojamez Junior Member

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    I have read that I can use a scanner or oscilloscope to check to see if the O2 sensor is bad. Is this something an auto repair shop can do, or could my local auto pets store (O'Reilly/AutoZone) could assist with?
     
  2. MADDOG

    MADDOG Super Moderator Staff Member

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    Most auto parts stores can read codes and give you a short printout.
     
  3. Kenworth

    Kenworth Junior Member

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    Repair shop can easily scan and fix. OBD II code readers are fairly cheap to buy and would diagnose many P codes.

    An O2 sensor is not difficult to switch out depending on its location and wether its siezed. If its siezed a little heat with a torch can free it up usually.
     
  4. cuevrojamez

    cuevrojamez Junior Member

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    Scanner is reading 2 P0420 codes, I assume 1 for the sensor before and 1 sensor after the cat converter. Is there a way to determine which sensor is bad?
     
  5. Bullitt5339

    Bullitt5339 Senior Member

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    The P0420 code is typically only for the sensor after the catalytic converter. It is not an "O2 sensor bad" code, it is a catalyst efficiency code which is normally the O2 sensor downstream from the catalytic converter, but it can also be caused by a bad catalytic converter, vacuum leaks, etc.

    You can test them with an O-scope. The voltage range should be between .2-.7 volts and if the cat is in good condition, it should stay pretty close to .5 volts after the engine has warmed up fully. You can try to use a voltmeter, but O2 sensors can switch voltage very rapidly and most voltmeters can't read the changes that fast, so it will average them out.
     

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