A Yamaha whose battery is flat after every second ride has a charging problem until proven otherwise, and the voltage regulator is the usual place to look. A single FX140 owner reports a corroded regulator causing an alarm and starting trouble, and for the VX1100 the same weak spot is known. Those are the two named cases, but every Yamaha four-stroke has a regulator and a battery, and the check is the same on all of them.
Read the voltage with the engine running
The M-JET Diagnostic shows battery voltage in live data on every supported model, which means the FX, VX, GP, EX and FZ series, the SuperJet and the JetBlaster from 2002 on. Take three readings in this order: ignition on with the engine off, while cranking, and with the engine running at a steady 3000 rpm or so. A healthy battery rests at about 12.5 volts. During cranking it sags, and a sag that goes very low points at a tired battery or a bad terminal. With the engine running the voltage should climb clearly above the resting value. If it stays at the resting figure, nothing is charging.
That last reading is the telling one. A voltage that stays at the resting level with the engine running is a charging problem, and a new battery on that ski will be flat again within days.
The regulator and its connector
The regulator sits in a wet environment, and corrosion is the failure owners describe. It usually starts at the plug and the mounting surface. A voltage that wobbles, drops out above a certain rpm or recovers when you move the harness points at a contact problem rather than at the part itself. If you can reach the regulator plug, disconnect it, check for green or white deposits, clean the contacts and seat it again. That costs nothing and sometimes restores the charging voltage. A regulator corroded inside won't be fixed that way.
Check the battery terminals and the main cables while you are at it. A loose or corroded terminal makes a healthy battery behave like a dead one and can pass for a regulator fault.
Which codes mean battery voltage
This is where the generation matters. On generation B, the VX1100 from 2005 and everything newer, code 19 is the battery voltage code. On generation A, the FX140, FX1000, FX1100 and GP1300R, battery voltage is code 25, and 19 means the pulser coil instead. Numbers differ between sources, so read the description in the app rather than trusting a figure from a thread. A battery voltage code at start-up can also be a weak battery that sagged while cranking, so the reading under load tells you more than the code alone.
What the scan can't tell you
The app tells you what the voltage is and whether the control unit complained about it. It can't see inside the regulator, the generator side or the battery. A low charging voltage fits a bad regulator, a fault on the generator side or a poor connection, and the live value doesn't separate those on its own. It narrows the field, and measuring at the regulator plug with a multimeter finishes the job. A battery that reads fine at rest can still collapse under load, so a load test or a proper charge cycle is the sensible next step.
The page on checking battery voltage and the charging system has more on the values. Before ordering parts, start with the terminals, then the voltage with the engine running, then the regulator plug.
