Yamaha YZ450FP 2023 Owner's Service Manual page 326

Fault code No.
Item
1 Mounted condition of throttle
position sensor.
Check for looseness or pinch-
ing.
Check the mounted condition
for correctness.
2 Defective throttle position sen-
sor.
3 Faulty ECU.
4 Delete the fault code.
Fault code No. 21
TIP
• Make sure that the machine is completely cold before checking the coolant temperature sensor.
• Before troubleshooting, disconnect the starter motor lead from the starter relay.
Fault code No.
Item
Fail-safe system
Diagnostic code No.
Diagnostic tool display
Procedure
Probable cause of malfunc-
Item
tion and check
16
Throttle position sensor: stuck throttle position sensor is detected.
Improperly mounted sensor 
Remount or replace the sensor.
Refer to "ADJUSTING THE
THROTTLE POSITION SEN-
SOR" on page 7-8.
Execute the diagnostic mode.
(Code No. 01)
Correct display range:
When the throttle is fully closed,
11–14 displays.
When the throttle is fully
opened, 86–92 displays.
Incorrect display range 
Replace the throttle position
sensor, and then reset the ECU
using the diagnostic code num-
ber "87".
Refer to "DIAGNOSTIC CODE:
SENSOR OPERATION TABLE"
on page 9-12.
Replace the ECU.
Confirm that the fault code has
a condition of "Recovered"
using the Yamaha diagnostic
tool, and then delete the fault
code.
21
Coolant temperature sensor: open or short circuit detected.
Able to start engine
Able to drive vehicle
06
Displays the coolant temperature.
Compare the actually measured coolant temperature with the Yamaha
diagnostic tool display value.
Maintenance job
8-27
FUEL INJECTION SYSTEM
Start switch to ON (no need to
start the engine), and open and
close the throttle several times.
Check the condition of the fault
code using the malfunction
mode of the Yamaha diagnostic
tool.
Condition is "Recovered"  Go
to item 4 and finish the service.
Condition is "Detected"  Go to
item 2.
Start switch to ON (no need to
start the engine), and open and
close the throttle several times.
Check the condition of the fault
code using the malfunction
mode of the Yamaha diagnostic
tool.
Condition is "Recovered"  Go
to item 4 and finish the service.
Condition is "Detected"  Go to
item 3.
Service is finished.
Confirmation of service com-
pletion