Generac Generator Alarm: Under Speed — Fix

What Does This Error Mean?

Generac Alarm: Under Speed means the engine RPM fell below the required 3,000–3,600 RPM range needed to maintain stable 60 Hz frequency. The generator shuts down to prevent under-frequency power reaching sensitive appliances. Common causes are a heavy overload, dirty air filter, fuel issue, or a governor problem.

Affected Models

Generac Guardian Series
Generac PowerPact Series
Generac Protector Series
Generac GP Series

Common Causes

Connected load is heavier than the engine can sustain, dragging RPM down
Air filter is clogged, restricting airflow and reducing engine power output
Fuel delivery problem — low fuel level, clogged fuel filter, or a stuck carburetor
Governor not responding correctly, failing to increase fuel delivery under load
Spark plug fouled or worn, causing weak combustion and low power

How to Fix It

Reduce the connected load

Disconnect some appliances and see if the engine speed recovers. Under Speed most commonly occurs when the load exceeds what the engine can sustain. Reduce total wattage to within 80% of the generator's rated capacity.
Check and clean or replace the air filter

A clogged air filter chokes the engine. Remove the air filter cover (usually a panel or wing nut on the side of the engine) and inspect the filter element. If it is grey or visibly dirty, replace it. Air filters on standby generators should be replaced annually.
Check the fuel level and quality

Ensure the fuel tank is adequately filled. On liquid-propane or natural gas generators, check that the gas shutoff is fully open and the gas supply pressure is correct. On gasoline models, old or phase-separated fuel (stored for 6+ months without stabilizer) reduces power output.
Inspect the spark plugs

A fouled spark plug causes incomplete combustion and reduces available power. Locate the spark plug(s), remove them, and inspect for carbon deposits or a worn electrode. Replace with the manufacturer-specified plug if they show wear.
Have the governor and carburetor inspected

If the above steps do not resolve the alarm, the governor linkage may need adjustment, or the carburetor jets may be partially blocked. These require a qualified Generac service technician to diagnose and adjust correctly.

Frequently Asked Questions

Is Under Speed the same as the generator being overloaded?

They are related but different. Overload occurs when the electrical output exceeds the rated watts and the alternator is stressed. Under Speed occurs when the engine cannot maintain the RPM needed for correct frequency — often triggered by an overload, but also by fuel or engine problems independent of load.

Can Under Speed damage my appliances?

Low-frequency power (below 60 Hz) can cause motors and compressors to run hot or slow. The Generac protection system shuts the unit down before under-frequency power causes damage, so a prompt shutdown is the correct response. Appliances connected during a brief Under Speed event are generally not harmed.