Possible causes of U3239 include a faulty oxygen sensor, faulty electrical connector at the oxygen sensor, faulty or shorted wiring in the oxygen sensor circuit, a blown fuse for the oxygen sensor (if applicable), low or excessive fuel pressure, a vacuum leak on the engine, excessive exhaust leaks, or a faulty PCM.
A sure sign of this is smoke or steam being emitted from underneath the hood. The ignition timing could be set incorrectly and will require adjustment. If, when looking under the hood, you notice that the drive belts appear loose, they need to be tightened or completely replaced. If there is a noticeable pool of fluid underneath the car, this could be a sign of the cylinder head gasket failing. That will be confirmed in the form of billowing smoke being blown from the exhaust system. Parts or components should not be replaced with reference to only a U3239 DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.
A repair technician can do so once repairs have been made.
The OBD II can automatically turn it off when it fails to detect the problem after several diagnostic cycles.
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