DF623 the main thing to check is to verify the operation of the VCT solenoid. You're looking for a sticking or stuck VCt solenoid valve caused by contamination. Refer to vehicle specific repair manual to perform component tests for the VCT unit.DF623 OBDII Repair :
If your vehicle failed a DF623 test and the DF623 Check Engine Light is NOT on, chances are you have a problem with the OBDII system, a burned out MIL lamp, or a faulty catalytic converter. The converter is essentially an afterburner that cleans up the exhaust after it exits the engine. The OBDII system uses a ownstream oxygen sensor to monitor the efficiency of the converter, and it should detect a drop in converter efficiency if the converter has been contaminated or is failing (ignition misfiring, leaky exhaust valves, and oil burning can all damage the converter). What you want to look any conditions that might cause ignition misfire, an overly rich or lean fuel condition, or loss of compression. Use your OBD Express DIY to look at the oxygen sensor outputs, coolant temperature, airflow, calculated engine load, and inlet air temperature.
Evaporative emissions systems test for a leak in the fuel tank and associated hoses. To perform the test, the computer pulls a vacuum and checks to see if the vacuum holds. Each car maker uses a different techniques to check, but most often it's some type of fuel tank pressure sensor.
Brake Switch Out of Self-Test Range Indicates that during Key On Engine Off (KOEO) Self-Test, BPP signal was high. Or during Key On Engine Running (KOER) Self -Test, the BPP signal did not cycle high and low. Open or short in (BPP) circuit Open or short in stoplamp circuits Damaged PCM Malfunction in module(s) connected to BPP circuit (Rear Electronic Module [REM] Windstar and LS6/LS8 and Lighting Control Module (LCM) Continental and Town Car) Damaged Brake Switch Misadjusted Brake Switch Check for proper function of stoplamps. Follow correct Self-Test procedures, refer to Section 2, Quick Test.
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