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 playClutch installation
Install the primary driven gear assembly (1).
Caution
if it is difficult to install the primary driven
gear, rotate the crankshaft.
Be sure to engage the oil pump drive
sprocket with the primary driven gear.
Install the spacer (2) and bea ...
DTC “c21” (p0110-h/l): iat sensor circuit
malfunction
Detected condition and possible cause
Detected condition
Possible cause
C21
Output voltage is not with in the following
range.
0.15 V ≤ Sensor voltage < 4.85 V
Iat sensor circuit open or short.
Iat sensor malfunction.
Ec ...
Voltage check
If voltage is supplied to the circuit being checked, voltage
check can be used as circuit check.
With all connectors/couplers connected and voltage
applied to the circuit being checked, measure
voltage between each terminal and body ground.
If measurements were taken as shown ...