Yamaha YZ 2019 Series Owner's Service Manual page 330

Fault code No. 21
TIP
• Make sure that the engine is completely cool 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
1 Connection of coolant tempera-
ture sensor coupler.
Check the locking condition of
the coupler.
Disconnect the coupler and
check the pins (bent or broken
terminals and locking condition
of the pins).
2 Connection of wire harness
ECU coupler.
Check the locking condition of
the coupler.
Disconnect the coupler and
check the pins (bent or broken
terminals and locking condition
of the pins).
3 Wire harness continuity.
4 Installed condition of coolant
temperature sensor.
Check for looseness or pinch-
ing.
5 Defective coolant temperature
sensor.
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
Improperly connected  Con-
nect the coupler securely or
repair/replace the wire harness.
Improperly connected  Con-
nect the coupler securely or
repair/replace the wire harness.
Open or short circuit  Replace
the wire harness.
Between coolant temperature
sensor coupler and ECU cou-
pler.
green/white–green/white
black/blue–black/blue
Improperly installed sensor 
Reinstall or replace the sensor.
Execute the diagnostic mode.
(Code No. 06)
When engine is cold:
Displayed temperature is close
to the ambient temperature.
The displayed temperature is
not close to the ambient temper-
ature  Check the coolant tem-
perature sensor.
Replace if defective.
Refer to "CHECKING THE
COOLANT TEMPERATURE
SENSOR" on page 8-66.
8-31
FUEL INJECTION SYSTEM
Confirmation of service com-
pletion
Push the start switch.
Fault code number is not dis-
played  Service is finished.
Fault code number is displayed
 Go to item 2.
Push the start switch.
Fault code number is not dis-
played  Service is finished.
Fault code number is displayed
 Go to item 3.
Push the start switch.
Fault code number is not dis-
played  Service is finished.
Fault code number is displayed
 Go to item 4.
Push the start switch.
Fault code number is not dis-
played  Service is finished.
Fault code number is displayed
 Go to item 5.
Push the start switch.
Fault code number is not dis-
played  Service is finished.
Fault code number is displayed
 Go to item 6.