Yamaha FI Code 21: Coolant Temp Sensor — Causes & Fix
On Yamaha motorcycles and scooters, FI fault code 21 means a fault was detected in the coolant temperature sensor circuit — the sensor the ECU uses for cold-start enrichment and fan control.
- Applies to
- Yamaha motorcycles
- Est. repair cost
- $15 – $90
- Time to fix
- 30–90 minutes
What it means
Common causes are a loose/corroded connector, low coolant around the sensor, damaged wiring, or a failed sensor. This is a generic Yamaha fuel-injection (FI) fault code, so it applies across Yamaha FI models — scooters and underbones like the Mio, NMAX, Aerox, and Sniper, plus YZF-R and MT motorcycles. Yamaha shows the code as a number on the diagnostic/odometer display, and the engine-warning light comes on. Once the fault is repaired and the ECU no longer detects it, the code clears; cycling the ignition (or using a Yamaha diagnostic tool) may be needed to confirm.
Symptoms
- FI code 21 with warning light on
- Hard cold starting or rough warm-up
- Cooling fan behaving oddly
How to fix it
- 1
Check the connector and coolant level
1. Locate the coolant temperature sensor. 2. Reseat the connector; confirm the coolant is full so the sensor stays submerged.
- 2
Test resistance and replace
1. Compare sensor resistance to the temperature chart. 2. Replace if out of spec, then clear the code.
Frequently asked questions
Can low coolant trigger code 21?
Yes — an uncovered sensor tip reads wrong. Top up and bleed the cooling system before replacing parts.
Last updated July 15, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.