Diagnostic Test Code (DTC) P0496 means that there is purge flow through the Evaporative Emissions System (EVAP) during non-purge. The Powertrain Control Module (PCM) measures fuel tank pressure by reading the voltage from the Fuel Tank Pressure Sensor (FTP). The PCM controls the pressure by opening and closing the EVAP Canister Purge Solenoid Valve (CPV). It accomplishes this by grounding and un-grounding the control circuit to the CPV. When the CPV is open, engine vacuum is applied to the EVAP canister and ultimately, the fuel tank itself. When this happens the pressure inside the tank drops, causing a voltage change in the FTP sensor.
This code sets when there is a pressure drop in the tank when the CPV is commanded closed (there should not be any vacuum applied).
The code can actually be caused by several things. It could be caused by a faulty FTP sensor giving false readings to the PCM. It can be caused by a voltage drop in the FTP sensor circuit caused by wire corrosion or a bad/corroded connector, etc. It can be caused by a short to ground in the CPV control circuit, causing the CPV to be "ON" all the time. It can be caused by a CPV that is just plain stuck and will not close properly.
A faulty or "stuck" CPV is the most likely cause of the problem however, the rest of the circuits and sensor involved should be checked for proper operation before replacing any parts. (It would be a total waste of your money to replace the CPV only to find that you have a shorted control circuit.)
P0496 = Evaporative Emission System High Purge Flow
Check this information about diagnostic: autoshop101.com/forms/h62 (pdf format)
Good luck.
SOURCE: 2002 Chevy Trailblazer Engine Codes
This is actually an easy swap out. Remove the electrical connection, remove the retaining bolt and pull out old one. I believe the sensor is located in the front passenger side of the engine on this particular model.
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