A 2004 FX HO that suddenly won't start, or starts and then sounds an alarm, leaves three suspects: the battery, the charging side with its regulator, and the ECU. The ECU gets blamed first because it is the part nobody wants to buy, and it is the least likely of the three. This applies to the early four-cylinder MR-1 family: FX140 and FX1000 (998 cc, 2002 to 2005), FX1000(A), FX1100 and FX HO MR-1 (1052 cc, 2003 to 2008). The M-JET Diagnostic supports all of them.
Voltage first, in three readings
Battery voltage is one of the live values in the app, and a weak battery is known to cause odd faults on these skis, so it is the cheapest place to start.
- Ignition on, before cranking. If the number is low, charge the battery and try again before looking at anything else.
- While cranking. A healthy battery sags but recovers quickly. A value that collapses points to the battery itself or a bad cable end.
- Engine running. The voltage should rise clearly above the resting value. If it stays where it was, the charging circuit is not delivering.
The last reading is where the regulator shows up. One FX140 owner traced an alarm and starting problems to a corroded voltage regulator, and for the VX1100 the same weak spot is known. The FX case is a single report, so treat it as a lead, not a rule. Open the connector and the regulator housing and look for green or white crust.
Coil pairs and injectors
The early FX does not use one coil per cylinder. Two coils each fire a pair of cylinders, 1 and 4 on one, 2 and 3 on the other. A dead coil therefore takes out two cylinders at once, which can look like a total no-start rather than a rough run. The component test in the app works the two coils, the four injectors and the fuel pump one at a time. Do it with good ventilation and no open flame or sparks nearby, since fuel is involved.
The pump test needs a caveat. It only shows that the pump runs. It does not show that the pump delivers enough fuel, so a pump that spins and a ski that still won't start is not a contradiction.
Codes that fit a no-start
On this generation the pulser coil is code 19 and the camshaft sensor is code 36. A missing signal from either fits a ski that cranks and does not fire. Code 71 is the slant switch, relevant if the ski was tipped or capsized, and code 25 is battery voltage. These numbers belong to the older control unit, and forum lists sometimes show different ones, so rely on the description the app prints beside the number.
What the scan can't tell you
It cannot declare an ECU dead. If the app will not connect at all with the ignition on, that points first to the supply side: battery cable, fuses, the connector in front of the ECU. It does not convict the control unit. A ski that connects, shows sane voltage, passes every coil and injector in the test and still does not run has a mechanical or fuel delivery question, and neither compression nor a weak pump will show up in a scan.
Start by reading the stored fault codes before you touch anything, then work through the component and actuator tests. Buy parts last.
