Yamaha YZ450FN 2022, Owner'S Service Manual, Page: 306 / 366

FUEL INJECTION SYSTEM
8-25
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.
1 Mounted condition of throttle
position sensor.
Check for looseness or pinch-
ing.
Check the mounted condition
for correctness.
Improperly mounted sensor

Remount or replace the sensor.
Refer to “ADJUSTING THE
THROTTLE POSITION SEN-
SOR” on page 7-8.
Start switch to ON (main relay
ON) 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.
2 Defective throttle position sen-
sor.
Execute the diagnostic mode.
(Code No. 01)
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.
Start switch to ON (main relay
ON) 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.
3 Faulty ECU. Replace the ECU. Service is finished.
4 Delete the fault code. Confirm that the fault code has
a condition of “Recovered”
using the Yamaha diagnostic
tool, and then delete the fault
code.
Fault code No. 21
Item Coolant temperature sensor: open or short circuit detected.
Fail-safe system
Able to start engine
Able to drive vehicle
Diagnostic code No. 06
Diagnostic tool display Displays the coolant temperature.
Procedure
Compare the actually measured coolant temperature with the Yamaha
diagnostic tool display value.
Item
Probable cause of malfunc-
tion and check
Maintenance job
Confirmation of service com-
pletion
1 Connection of coolant tempera-
ture sensor coupler.
Check the locking condition of
the coupler.
Disconnect the coupler, and
check the pins (for bent or bro-
ken terminals and locking condi-
tion of the pins).
Improperly connected

Con-
nect the coupler securely, or
repair/replace the wire harness.
Start switch to ON (main relay
ON), and then check the condi-
tion of the fault code using the
malfunction mode of the
Yamaha diagnostic tool.
Condition is “Recovered”

Go
to item 7 and finish the service.
Condition is “Detected”

Go to
item 2.
Fault code No. 16
Item Throttle position sensor: stuck throttle position sensor is detected.