A fault determining apparatus for a variable valve timing mechanism is provided
for shortly and accurately determining a fault even at low oil temperatures. The
mechanism is driven by an oil pressure to change a cam phase of an intake cam and/or
an exhaust cam with respect to a crank shaft of an internal combustion engine,
thereby controlling the actual cam phase to reach a target cam phase. The apparatus
has a fault determining routine executed by an ECU (electric control unit) for
determining that the mechanism is faulty when a cam phase difference between the
target cam phase and the actual cam phase is not within a predetermined range,
an oil temperature sensor for detecting the temperature of the working oil, and
a correcting routine executed by the ECU for correcting the predetermined range
in an increasing direction when the detected oil temperature is low.
