The 1.8-litre Yamahas carry three temperature devices that can all end in the same overheat alarm, and which one complained is not obvious from the saddle. The 1.8 family means the VXR and VXS, the FX HO and Cruiser HO of 2008 to 2014, the FZR and FZS, the FX SVHO and the GP1800 series. The three-cylinder TR-1 models, meaning the VX from 2015 and 2016, the EX, the SuperJet and the JetBlaster, report codes 15 and 65 as well, and owners go about them the same way. The 1.9-litre FX, GP and VX of 2024 add a further entry, code 41.
Who is who
- The thermo sensor on the exhaust manifold measures the water temperature there. A fault shows up as code 65.
- The engine temperature sensor reports the engine itself and has code 15.
- The thermo switch is a separate device. It gives no number to read, and a bad one may leave no code in memory at all.
- On the 1.9-litre engines a thermo sensor message appears as code 41. Those engines are naturally aspirated.
Numbers can differ between control unit generations and between sources, so treat the description in the app as the reference.
What live data tells you
The M-JET Diagnostic supports all the models above and lists engine temperature and, where the model has it, the exhaust manifold temperature among up to 34 live values. Warm the engine up on the water, never dry on land, and watch both curves. After the warm-up they should rise and then level off. A value stuck at an impossible figure, jumping around or never moving points at that sensor or its connector. If both curves look reasonable and the alarm still sounds, owners begin to suspect the thermo switch, and the reasoning holds: when no sensor reports anything, the switch is the device left over.
Flow comes before sensors
The overheat story is reported again and again on EX, VX, FX SVHO and FZR models, and the usual verdict is a water path problem before a sensor problem. The candidates are cavitation at the pump, foreign matter in the intake grate, a damaged impeller, a plugged small cooling-water passage, and manifold passages clogged after poor flushing. A popular check is the test tube at the front: if water leaves at the rear and nothing comes out at the tube, the path is closed somewhere. On the EX, reports say it shows on long rides and hardly at all on calm water.
A diagnostic tool sees none of this. Cavitation, impeller wear and clogged passages don't set codes. What you do see is a pattern. A temperature that climbs steadily over a long run while the sensors behave plausibly points at flow, and a value that is flat or erratic from the moment you switch on points at the sensor.
A warning tone with two meanings
On some Yamaha skis the overheat warning and the oil pressure warning share the same tone. According to the EX manual both warnings reduce engine speed automatically, and the manual names cleaning the intake and letting the engine cool as the remedy. So a beep plus lost rpm doesn't tell you which of the two fired. Read the codes and the live values at that moment, or from a logged ride, before you decide.
There is more on this in the page about overheating and temperature sensors. A sensible order is intake grate and water outlet first, then the live curves, then connectors, and a new sensor or switch last.
