You bought a Generac standby generator so you would never have to worry about power outages. But now the power is out, the lights are off, and your generator is sitting silently in the yard doing nothing. Few things are more frustrating than a generator that refuses to start when you need it most. Before you panic, many Generac start failures have simple causes that you can identify yourself, even if the repair requires a professional. Here is a systematic guide to the most common reasons a Generac will not start and what to do about each one.
Check the Basics First
Before diving into troubleshooting, verify these fundamental items that account for a surprising number of service calls:
Is the Generator in AUTO Mode?
The most common non-mechanical reason a Generac does not start during an outage is that it has been left in OFF or MANUAL mode. Check the control panel on the generator. The main switch should be set to AUTO for the generator to start automatically when the power goes out. If someone switched it to MANUAL for maintenance or testing and forgot to switch it back, the generator will not respond to an outage.
Is the Circuit Breaker On?
Check the main circuit breaker on the generator itself. If it has tripped, the generator may not start or may start but not deliver power. Also check the circuit breaker that feeds the battery charger. If this breaker has tripped, the battery charger cannot keep the battery topped off, and the battery may be too weak to start the engine.
Is There Fuel?
For propane generators, check the tank level gauge. An empty propane tank is an obvious but overlooked cause. Also verify that the manual shutoff valve on the fuel line is in the open position. For natural gas generators, confirm that your gas service is active and that no other gas appliances in the house are affected.
Dead or Weak Battery
A dead battery is the single most common cause of Generac start failure. The battery provides the electrical power to crank the starter motor, and without adequate cranking power, the engine cannot start.
Symptoms
- The generator cranks slowly or not at all when it tries to start
- You hear a clicking sound from the starter solenoid but no cranking
- The control panel display is dim or blank
- The generator has been showing a "Battery Low" warning on the Mobile Link app
What You Can Do
If you have a multimeter, check the battery voltage. A fully charged battery should read 12.6 volts or higher. Below 12.0 volts, the battery likely cannot start the engine. You can attempt to jump-start the generator using a car battery and jumper cables: connect positive to positive and negative to a ground point on the generator frame (not the negative battery terminal) to avoid sparks near the battery.
If the generator starts with a jump, the battery needs replacement. Do not rely on jump-starting as a long-term solution. A failed battery will only get worse.
When to Call a Pro
If the battery is new and still not holding a charge, the problem may be the battery charger, the charging circuit, or a parasitic drain. These require diagnostic equipment and electrical testing to identify. Call a generator technician to diagnose and repair the charging system.
Low Oil Pressure
All Generac generators have a low oil pressure safety switch that prevents the engine from starting if the oil level is too low. This protects the engine from catastrophic damage, but it also means the generator will refuse to start if the oil is even slightly below the minimum level.
Symptoms
- The generator cranks but does not start, or starts briefly and shuts down immediately
- The control panel shows a "Low Oil" or "Low Oil Pressure" alarm
- The engine does not even attempt to crank (some models prevent cranking when oil is critically low)
What You Can Do
Check the oil level using the dipstick. If the oil is low, add the correct type and quantity specified in your owner's manual. Most Generac air-cooled generators use 5W-30 full synthetic oil. After adding oil, you may need to reset the low oil alarm on the control panel before the generator will attempt to start.
To reset most Generac controllers: turn the main switch to OFF, wait 30 seconds, then turn it back to AUTO. The alarm should clear if the oil level is now adequate.
When to Call a Pro
If the oil level is correct but the low oil alarm persists, the oil pressure sensor or sending unit may be faulty. These components can fail over time and give false low-pressure readings. A technician can test the actual oil pressure with a mechanical gauge to determine whether the problem is real or sensor-related.
Fuel System Problems
Fuel system issues are the second most common category of Generac start failures after battery problems. The fuel system includes the gas supply, fuel solenoid valve, regulator, and carburetor or fuel injection system.
Symptoms
- The engine cranks normally but does not fire
- The engine starts briefly, sputters, and dies
- The engine runs but surges (hunts up and down in RPM)
- You smell gas when the generator is trying to start (indicates fuel is flowing but not igniting properly)
What You Can Do
Verify that the fuel supply is available and the manual shutoff valve is open. For propane systems, check that the tank has at least 20 percent fuel. Propane systems can struggle to deliver adequate fuel pressure when the tank is nearly empty, especially in cold weather when propane vaporization slows down.
Listen for the fuel solenoid clicking when the generator tries to start. You should hear a distinct click from the solenoid valve opening. If you do not hear it, the solenoid may have failed or may not be receiving the signal to open.
When to Call a Pro
Fuel system repairs on Generac generators require specialized knowledge. The fuel regulator, solenoid valve, and carburetor are precision components that must be adjusted correctly for safe operation. Improper adjustments can cause fuel leaks, which are a fire and explosion hazard. Always have a qualified generator technician handle fuel system repairs.
Control Board Errors
The control board is the brain of the Generac generator. It manages starting sequences, monitors sensors, controls the transfer switch, and logs fault codes. When the control board detects a problem, it locks out the generator to prevent damage.
Symptoms
- The control panel displays a fault code (such as RPM Sensor Loss, Overcrank, Overspeed, or Undervoltage)
- The generator made three start attempts and locked out (Overcrank alarm)
- Warning lights are illuminated on the control panel
- The Mobile Link app shows an alarm status
Common Generac Fault Codes
- Overcrank (Code 1100): The engine cranked but did not start within the allowed time. Usually caused by fuel problems, dead battery, or spark plug failure
- Overspeed (Code 1200): The engine RPM exceeded safe limits. May indicate a governor problem or load rejection
- Low Oil Pressure (Code 1300): Oil level or pressure below minimum. Check oil level first
- High Temperature (Code 1400): Engine overheating. Check coolant level, blocked airflow, or excessive load
- RPM Sensor Loss (Code 1501/1505): The controller lost the engine speed signal. Often a faulty speed sensor or wiring issue
- Undervoltage (Code 1600): Generator output voltage too low. May indicate a voltage regulator failure
- Overload (Code 2100): Generator is being asked to produce more power than it is rated for
What You Can Do
For many alarm codes, you can attempt a manual reset. Turn the generator's main switch to OFF, wait at least 30 seconds, then turn it back to AUTO. If the underlying problem has been resolved (such as adding oil for a low oil alarm), the generator should return to standby mode and be ready to start.
For an Overcrank alarm (the most common fault code), try resetting once. If the generator overcranks again after reset, there is a persistent problem preventing the engine from starting, and further troubleshooting is needed.
When to Call a Pro
Any fault code that recurs after reset requires professional diagnosis. Control board issues, sensor failures, and wiring problems require diagnostic tools, wiring diagrams, and experience with Generac systems. Attempting to bypass safety systems to force the generator to run can cause engine damage, fire, or electrical hazards.
Transfer Switch Problems
The automatic transfer switch (ATS) is what tells the generator to start and what connects your home to generator power. If the transfer switch fails, the generator may not receive the start signal at all, or it may start but fail to deliver power to your home.
Symptoms
- The power goes out but the generator does not attempt to start
- The generator starts and runs but your house remains without power
- You hear the generator running but only some circuits have power
- The transfer switch is making unusual buzzing or humming sounds
What You Can Do
Check the transfer switch for any visible issues. On Generac Nexus and Evolution controllers with integrated transfer switches, look for fault codes on the controller display. Check that all wiring connections to the transfer switch are secure and that no breakers inside the transfer switch panel have tripped.
When to Call a Pro
Transfer switch repair involves working with high-voltage electrical components that can cause electrocution. Never attempt to open, service, or bypass an automatic transfer switch unless you are a licensed electrician. The transfer switch handles both utility power (which may be live even during an outage on your side) and generator power simultaneously. This is not a DIY repair under any circumstances.
When to Call a Professional
As a general rule, homeowners can safely perform these troubleshooting steps: checking the mode switch, checking oil level and adding oil, checking fuel supply and valve positions, checking battery voltage, and resetting fault codes. These involve no exposure to high-voltage components and minimal risk.
You should call a professional for: any recurring fault code, fuel system repairs, transfer switch issues, wiring problems, control board replacement, any situation where you smell gas, and any situation where the generator is running but producing abnormal sounds, smoke, or excessive heat.
Unity Power & Light provides Generac generator repair and maintenance throughout the Aiken area. Our technicians carry common replacement parts including batteries, spark plugs, filters, solenoid valves, and sensors, so most repairs can be completed in a single visit. Call us at (803) 220-4491 for same-day or next-day generator service.