Yamaha YZ 2020 Series Owner's Service Manual page 316

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
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).
2 Connection of wire harness
ECU 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).
3 Wire harness continuity.
4 Mounted condition of coolant
temperature sensor.
Check for looseness or pinch-
ing.
Check the mounted position for
correctness.
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 mounted sensor 
Remount or replace the sensor.
8-29
FUEL INJECTION SYSTEM
Confirmation of service com-
pletion
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.
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 3.
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 4.
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 5.