Owning and maintaining Mercury Marine L6 300 engines means that every once in a while you are going to run into an alarm that seems much more serious than it actually is. Recently, at 911 engine hours, we experienced an alarm on our port engine during completely normal operating conditions that initially pointed us toward a possible electrical issue.
The alarm displayed:
“Alarm – Port Engine. Voltage
Sensor Power Supply Voltage is low.
(Non-Critical – Service Engine Soon.)”
As soon as the warning appeared, the boat was shut down and restarted to see if the fault would clear itself. Unfortunately, the alarm immediately returned after restart and continued to stay active. Anytime an engine throws a voltage-related warning, especially on modern electronically controlled outboards, it is easy to assume the issue could involve batteries, charging systems, ECU problems, or wiring faults.
What made the situation confusing was that the engine itself continued running perfectly fine. There were no drivability issues, no overheating, no loss of power, and no charging abnormalities noticed during the ride home. The only additional symptom was that the trim gauge information for the port engine disappeared entirely from the MFD. The engine could still be trimmed normally using the controls, but there was no trim position data being displayed.
That missing trim information ended up being the key clue.
After troubleshooting the issue further, the actual cause turned out to be a failed trim sender sensor. The faulty component was a Mercury trim sender, part number 8M0010626. On the Mercury L6 300, this sensor is located on the port side of the engine near the kickstand or tilt support bracket. It is secured with two T27 Torx bolts, making it relatively simple to access. The connector for the sender routes toward the backside of the engine near the fuel rail area.
Once the problem was identified, the repair itself was incredibly straightforward. After disconnecting power, the old sender was removed by taking out the two T27 bolts and disconnecting the electrical connector. The new sender was installed in its place, the connector was plugged back in, and the engine was restarted. Immediately, the trim data returned to the display and the alarm cleared.
What makes this issue particularly frustrating is how misleading the alarm message can be. A warning that references “Sensor Power Supply Voltage” naturally sends most people looking toward the charging system or electrical supply side of the engine. In reality, the failed trim sender was pulling down the sensor reference voltage enough to trigger the fault. Since these engines rely on shared reference voltages between multiple sensors, one failed component can sometimes create alarms that appear unrelated at first glance.
For anyone maintaining higher-hour Mercury L6 300 engines, this is definitely a problem worth keeping in mind. If you experience this exact alarm while also losing trim position data on your display, there is a very good chance the trim sender is the culprit. Considering how inexpensive and easy this repair is compared to chasing down larger electrical issues, it can save a significant amount of time and frustration.
In our case, replacing the sender only took a couple of minutes and immediately solved the issue completely.
