Yamaha Error Code 21: Coolant Temperature Sensor: Open or Short Circuit

The ECU has found a break or short in the coolant temperature sensor circuit, so it can no longer read the engine temperature and the fuel mixture may be wrong, especially on a cold engine.

Medium risk

The engine usually keeps running, but it may start or run poorly until it warms up. Fix it soon rather than riding on it for weeks.

What causes Yamaha error code 21

  • CommonSensor connector not fully seated or shaken loose
  • CommonBroken wire or poor earlier repair near the connector
  • PossibleFailed coolant temperature sensor

How to check and fix it

Start at the top: the most likely and usually cheapest checks come first.

  1. Sensor connector not fully seated or shaken loose. Find the coolant temperature sensor on the engine (it screws into a coolant passage), then unplug and firmly reconnect its connector until it locks. Vibration can work a half-seated connector loose while riding.
  2. Broken wire or poor earlier repair near the connector. Flex the wires gently where they enter the connector and look for a break, a cracked solder joint or loose tape from a previous fix. Repair the wire properly and secure it so it cannot flex at the connector.
  3. Failed coolant temperature sensor. Measure the sensor's resistance with the engine cold and again when warm, and compare with the values for your model. If you replace it, some coolant will escape, so top up the coolant afterwards and check for leaks.

Parts & tools you may need

  • Engine coolant
  • Digital multimeter
  • Cable ties

FAQ

Can I still ride with code 21?

Usually, yes, for a short while. The engine normally keeps running, but it may start hard or run poorly until it warms up because the ECU cannot read the engine temperature. Fix it soon rather than riding on it for weeks.

Do I always need a new sensor for code 21?

No. Owners have cleared code 21 just by reseating a connector that had worked loose, or by repairing a broken wire at the connector. Check the connection before buying parts.

Is code 21 used on air-cooled bikes?

Air-cooled models do not have a coolant sensor, and some use a different temperature sensor instead. Check what your model uses before following these steps.

Sources

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