Adjust the engine idle speed periodically on the engine at normal operating temperature.
To adjust the idle speed:

Engine oil and filter change
Throtile cable playDTC “c28” (p1655): secondary throttle
valve actuator (stva) malfunction
Detected condition and possible cause
Detected condition
Possible cause
The operation voltage does not reach the stva.
Ecm does not receive communication signal from the
stva. Stva can not operate properly or its motor locked
Stva malfunction.
...
Noise emission control system description
Tampering with the noise control system prohibited: local law or federal law
prohibits the following
acts or the causing thereof:
the removal or rendering inoperative by any person, other than for
purposes of maintenance, repair or
replacement, of any device or element of design in ...
DTC “c15” (p0115-h/l): ect sensor circuit
malfunction
Detected condition and possible cause
Detected condition
Possible cause
C15
Output voltage is not with in the following
range.
0.15 V ≤ Sensor voltage < 4.85 V
Ect sensor circuit open or short.
Ect sensor malfunction.
Ec ...