There are a few things that affect VTEC engagement:
RPM
Throttle %
Oil Press
Engine Temp
The things that use those parameters:
ECU
VTEC solenoid
VTEC pressure switch (oil)
Coolant Temp sensor
Make sure there is continuity between the ECU and VTEC solenoid.
Does the check engine light come on while driving?
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
Team B.O.B.® - Ballaz on a Budget