Question about 2001 Cadillac DeVille

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MY CHECK ENGINE LIGHT CAME ON AND I HAD THE AUTOPARTS CHECK IT THE CODES ARE ...U0156 AND.....P0420..... I KIND OF SMELL OIL BUT ..NO LEAK ..OR NO SMOKE.. I NEED YOUR HELP PLEASE THANK YOU

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Here is a link to the U0156 there is not a lot of information about the U0156. It is something to do with the ignition switch or some electrical running to the ignition switch.
http://www.aboutautomobile.com/TSB/2007/Cadillac/Escalade/Ignition+Switch

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
Possible Solutions 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.

Posted on Jun 26, 2011

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Engine check light read p0171 and p0420, noticed a leak of engine oil on cylinder 2 area of head gasket when checking the sparks plug, is that the cause?


P0171 code is described as: lean condition bank 1, oxy sensor 1. If your vehicle is a 4 cylinder, which I believe it is.. you have got a lean air/fuel mixture being found by the ecm(computer).This can be caused by several things. The first of which is a possible vacuum leak at or near the intake manifold. This can also be caused by a MAF(Mass Air Flow) sensor problem or even by something as simple as a clogged fuel filter. This lean condition could also be the cause of the P0420 :described as a catalyst efficiency problem. It would be best to have the vehicle taken to a reputable repair facility that is capable of engine performance diagnostics and have them run tests for these codes. The gasket you mention is most likely a valve cover gasket not the head gasket and would not cause these codes. Although you should have this leak fixed before it becomes a problem.

Hope this helps..

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