An owner of a 2020 EX describes an engine that starts and then stops by itself after about 15 seconds at idle. A diagnostic read found no codes, and the live values looked normal. As far as I can tell, no solution has been reported. That makes it a useful example for this article, because it is where diagnostics reaches its limit.
It is a single case, so don't treat it as a typical EX fault. It concerns the EX series from 2017, with the three-cylinder TR-1 engine, and the checks below apply to any of them.
What a clean scan really tells you
No codes and normal live values rule out a good deal: a dead sensor, a shorted circuit, a battery that is too weak and some switch faults. They don't rule out anything that the control unit has no sensor for. The control unit can only report on what it can measure, and a stall at idle with all signals plausible is a problem the measurements don't show.
What the app cannot see: how much fuel the pump delivers, the level of the oil, an air filter soaked with oil, a corroded plug that is intact at rest, spark plug condition, and loss of compression. Those are exactly the things to look at next.
Checks to do anyway
- Oil level. On the TR-1 engines, owners report overfilling after an oil change, with lost speed and oil in the air filter and intake. Check the level only after the engine has run about six minutes on level ground. If it is too high, extract the excess.
- The air filter box. If it is wet with oil, it is the problem or part of it.
- The idle and the throttle valve. After cleaning, or after the battery has been disconnected, the stored values can be off. The app can relearn both, and it costs a minute. It is not a repair, but it removes one doubt.
- Battery voltage under load and with the engine running. Normal resting voltage doesn't prove the battery holds up while the starter turns.
- The kill switch on the left handlebar. A corroded one is a known problem on the 2021 VX, where it can stop the engine or fail to stop it, and the switch state in the live data tells you whether it changes during the stall.
- The fuel pump, tested individually in the app. A pump that runs says nothing about flow. A weak pump can pass the test and still starve the engine.
Whenever you open up anything in the fuel system, work with good ventilation and no ignition sources around, and don't run the engine on land without water supplied to the cooling system.
Logging the stall
Run the engine to the point where it stalls, with the logger recording, and look at the seconds before it quits. If rpm sags first and intake pressure moves, you are looking at air or fuel. If battery voltage or a switch changes just before, look at the electrical side. If everything holds steady until it stops, the cause probably isn't something the control unit can measure.
The M-JET Diagnostic supports the whole EX series. It reads codes and live values, runs the individual coil and injector tests and the pump test, and relearns idle and throttle. The page on rough idle diagnostics covers the idle side of the problem, and the EX diagnostics page lists what the app shows per variant.
A scan that finds nothing narrows the field, and the mechanical checks are then where the answer is most likely to turn up.
