Underspeed

Overview

What 1600 / 1603 means

Engine speed dropped below threshold. Likely overload or fuel delivery issue.

Generator overloaded — too many appliances running simultaneously

Insufficient fuel pressure (low propane, gas pressure issue)

Clogged fuel filter or carburetor

Failing governor unable to compensate for load

Dirty air filter restricting airflow to engine

Engine losing compression due to wear

Large motor load startup surge (A/C compressor, well pump)

Step-by-step workflow

Follow the diagnostic and reset sequence

1

Underspeed during a power outage almost always means the generator is overloaded. Large motor loads (central A/C, well pumps, electric dryers) draw 2–3x their rated current at startup.

Immediately shed large loads: turn off the A/C at the breaker, disconnect large appliances. The generator rating is on the nameplate — compare to your connected load. Generac recommends keeping total load below 80% of rated output for sustained operation.

2

Low fuel pressure causes the engine to starve under load, resulting in RPM drop. This is especially common with propane during cold weather or when the tank is below 20%.

Propane: Confirm tank is above 25% and regulator is functioning. Cold weather significantly reduces propane vapor pressure. Natural gas: Confirm supply pressure at the generator — call your utility if pressure seems low.

3

A severely clogged air filter restricts airflow, causing the engine to run rich and lose power under load.

Remove and inspect the air filter. If visibly dirty or grey — replace it. Generac recommends air filter inspection every 200 hours or annually.

4

After shedding load and checking fuel, clear the fault and restart. Add loads back gradually — one at a time — to identify which appliance causes the underspeed.

Evolution: Press OFF/Reset. Nexus: Press OFF → Enter. Start with essential loads only. Avoid starting large motor loads simultaneously. If underspeed returns with minimal load, fuel delivery or governor issue is likely.

After reset

Did the code clear?

Yes — code cleared

The code cleared and the unit returned to standby. Log the incident, confirm ready-to-run status, and monitor the next exercise cycle.

No — code returned

If the code returns immediately, stop resetting and move into service. The issue likely needs inspection of wiring, switches, or a controller-level component.

Open a Service Ticket

Prevention

Best practices and next checks

Know Your Generator's Capacity

Review your connected load vs. generator rating. Keep total load below 80% of rated kW output.

Maintain Fuel Supply

Keep propane above 25%. For NG, have pressure tested annually. Low fuel pressure causes underspeed under load.

Replace Air Filter Annually

A clean air filter ensures the engine can breathe properly under full load.

Get a Maintenance Plan

A&A Power Generators will inspect your fuel system, governor, and load capacity. +1-888-991-6933

Stagger Large Load Starts

Use a load management strategy — don't start your A/C and well pump simultaneously.

Enable Mobile Link Alerts

Receive instant Underspeed notifications via the Generac Mobile Link app.

Get a Maintenance Plan

Let A&A Power Generators handle all preventative maintenance for your generator.

What 1600 / 1603 means

Follow the diagnostic and reset sequence

Check Connected Load

Check Fuel Supply

Inspect Air Filter

Clear Fault & Test with Reduced Load

Did the code clear?

Yes — code cleared

No — code returned

Move from troubleshooting into the right support path.

Best practices and next checks

Know Your Generator's Capacity

Maintain Fuel Supply

Replace Air Filter Annually

Get a Maintenance Plan

Stagger Large Load Starts

Enable Mobile Link Alerts

Get a Maintenance Plan

A&A Power can take it from here.

Safety Information

A&A guidance on demand