Read the manual thoroughly and understand all of the instructions, cautions, and warnings before using this equipment. If any section of the manual is not understood, contact your nearest authorized dealer, or contact Generac Customer Service at 1-888-436-3722 (1-888-GENERAC), or www.generac.com with any questions or concerns.  

My dual fuel or multi-fuel portable generator won’t start on liquid propane or natural gas

If your dual fuel or multi-fuel portable generator will not start on liquid propane or natural gas, the issue is often related to startup procedure, fuel selection, or fuel supply conditions. This article walks through the most common causes and troubleshooting steps to help you identify and resolve the problem before seeking service.

Environment

• Dual fuel, tri-fuel, or multi-fuel Generac portable generators
• Operation on gasoline, liquid propane (LP), or natural gas (NG)

Verify the startup procedure

Before troubleshooting further, confirm that the generator is being started exactly as outlined in the owner’s manual. An incorrect startup sequence is the most common cause of a no-start condition.

• Confirm the correct fuel type is selected (gasoline, LP, or NG): How to switch a dual fuel portable generator between gasoline and liquid propane
• Ensure the fuel valve is in the proper position
• Set the choke correctly (if applicable)
• Follow the proper order for turning the engine switch and pull-starting or using electric start

Refer to your specific model’s owner’s manual for detailed startup instructions.

Liquid propane (LP) troubleshooting

Confirm the propane tank size and type

When operating on liquid propane, only specific tank sizes and connections are supported.

• Use only a 20-pound or 30-pound LP cylinder
• The tank must have Type 1, right-hand Acme threads (standard propane grill style tank)
• Do not use tanks smaller or larger than the specified sizes

Using an incorrect tank size or connection type can prevent proper fuel flow and cause starting issues.

Verify fuel configuration settings

If your generator supports both gasoline and propane, it must be properly configured for the selected fuel.

Review the complete step-by-step checklist in the following article to ensure the generator is fully configured to the correct fuel:

How to switch a dual fuel portable generator between gasoline and liquid propane

Confirm all required switches, valves, and hoses are set correctly before attempting to start the generator.

Bleed the propane line

If the propane tank was previously connected to a grill or another appliance, air may be trapped in the hose.

• Securely connect the provided LP hose to the generator's inlet and the LP tank
• Verify the propane tank valve is fully open (rotate the valve counter-clockwise)
• Allow a few seconds for propane to flow through the hose before starting
• Attempt to start the generator again after bleeding the line

Air in the fuel line can prevent propane from reaching the engine during startup.

Fuel source troubleshooting

If the generator starts and runs on gasoline but will not start on propane, the issue is likely related to propane supply conditions.

• The propane tank may be too cold, reducing vapor pressure.
• The tank may not have sufficient fuel
• Low ambient temperatures can affect propane performance
• Try a different propane tank, allow the tank to warm naturally, or confirm adequate fuel pressure before retrying. 

If none of the steps above resolve the issue, the generator may require professional inspection or service.

Locate an authorized Generac dealer using the dealer locator.

An authorized dealer can diagnose fuel system components and ensure safe, proper operation.

Natural gas troubleshooting

If the generator starts and runs on gasoline or liquid propane but will not start on natural gas, the issue is likely related to the natural gas supply conditions. Follow troubleshooting steps below to verify the fuel source and fuel system components are adequate for generator operation:

Verify fuel pressure

Natural gas generators require a specific fuel pressure range to operate correctly.

• Recommended natural gas fuel pressure for dual fuel, tri-fuel, or multi-fuel Generac portable generators is 3.5–7.0 inches of water column at the generator fuel inlet
• Fuel pressure below specification can prevent starting or cause rough operation

Fuel pressure should be checked by a Generac IASD or licensed plumber using a manometer at the generator fuel inlet if starting issues persist.

Confirm proper pipe sizing

Undersized gas piping is a common cause of natural gas starting problems.

• Verify the gas supply line is sized correctly for the generator’s kW rating. Refer to your generator's owner's manual for NG pipe sizing requirements
• Generally speaking, longer pipe runs and additional fittings may require larger pipe diameters
• Improper pipe sizing can restrict fuel flow even if pressure appears correct at the source

Natural gas piping should only be installed by qualified service personnel. All gas piping must comply with local codes and applicable NFPA standards.

Inspect shutoff valves and connections

Multi-fuel generators will require an external manual fuel shutoff valve (A) on the fuel line. The external fuel shutoff valve must be installed within six (6) feet (1.8 m) of the generator fuel inlet

• Confirm the external manual fuel shutoff valve is fully open
• Inspect flexible fuel lines and quick-connect fittings for damage or leaks

Figure 2-18 illustrates a fuel shutoff valve with a manometer port for making fuel pressure checks. This optional accessory fuel shutoff valve permits making pressure checks for diagnostic purposes without going into the generator enclosure. 

Check for gas leaks or air in the line

Air in a newly installed or recently serviced gas line can prevent the generator from starting. Work with your fuel supplier to:

• Ensure the gas line has been properly purged
• Check all connections using approved leak detection methods

Fuel source troubleshooting

• If the generator starts and runs on gasoline or liquid propane but will not start on natural gas, the issue is likely related to the natural gas supply conditions
• Double check the above troubleshooting steps to verify the fuel source and fuel system components are adequate for generator operation

Contact a dealer for support

If none of the steps above resolve the issue, the generator may require professional inspection or service.

Locate an authorized Generac dealer using the dealer locator.

An authorized dealer can diagnose fuel system components and ensure safe, proper operation.