Overspeed Shutdown
The engine suddenly races well past its normal running speed, or the controller cuts the engine before you even notice the sound changing. Overspeed is a protective shutdown, not an inconvenience to reset around — an engine that's actually overspeeding is a genuine safety hazard.
What this looks like
A household standby or portable generator's engine is governed to hold a narrow speed band — typically around 3,600 RPM on a 60Hz 2-pole unit — because that speed is what keeps output frequency at 60 Hz. Overspeed protection exists because a governor or throttle failure can let the engine run away well above that, which can shatter alternator windings or throw a rod in seconds. You'll usually notice it one of two ways: either you hear the engine note climb and go rough right before it shuts down, or the shutdown happens fast enough that all you see is the fault. Generac's Evolution/Nexus controller reports this as code 1200; Kohler's RDC2/DC2 shows the plain-text fault EngineSpeed High, tripping once speed exceeds roughly 115% of normal for more than three-tenths of a second; Briggs & Stratton's Home Generator System blinks its LED six times, decoded as FC_6, and cuts power once frequency crosses about 70 Hz.
Most likely causes, ranked
- Sticking or binding throttle linkage or stepper motor. The mechanical link between the governor and the carburetor throttle plate can corrode, bend, or jam, especially on units that don't get exercised regularly.
- Failed electronic governor or speed sensor. Modern standby units use an electronic governor reading engine RPM; a bad sensor or governor board can command more throttle than the engine actually needs.
- Unstable or over-rich fuel mixture. A carburetor or mixer assembly issue can cause the engine to hunt for speed, sometimes overshooting badly enough to trip the fault.
- Linkage left loose or disconnected after recent service. If the unit was serviced or had its carburetor removed recently, a linkage arm that wasn't fully reconnected is a common, easily overlooked cause.
Which fault codes match this
| Brand | Code / display | Trigger threshold |
|---|---|---|
| Generac | 1200 (Overspeed) | Engine speed above the controller's safe governed limit |
| Kohler | EngineSpeed High | ~115% of normal running speed for more than 0.3 seconds |
| Briggs & Stratton | FC_6 (6 blinks) | Output frequency exceeding roughly 70 Hz |
What you can check yourself
- Visually inspect the throttle linkage and stepper motor arm for anything binding, disconnected, corroded, or obviously bent.
- Clear the fault at the controller and, from a safe distance and only if you're comfortable doing so, watch whether the engine surges or races on the next start attempt.
- Do not repeatedly force restarts if the engine visibly races — repeated overspeed events can damage alternator windings even if the engine itself survives.
- If the unit was serviced recently, check whether a linkage arm or governor spring was left loose.
More help
Confirmed the exact code already? Try the code lookup. Otherwise browse the full list for your brand: Generac, Kohler, or Briggs & Stratton.