Generac 1505 Error Code: RPM Sensor Loss

What It Means: Your Generac Evolution generator tried to start but the engine didn’t actually turn over, or it was running and suddenly lost RPM signal from the ignition system.

Understanding the 1505 Alarm

The 1505 RPM Sensor Loss alarm on Generac Evolution 1.0/2.0 units (Honeywell Sync 2.0/3.0 controllers) tells you that the generator’s control board sent a crank command to the starter, but never received confirmation that the engine was actually spinning. This is a safety feature—the controller is essentially saying, “I told the engine to start, but I didn’t hear back that it was working.”

On two-cylinder models, this alarm fires during the cranking phase. On single-cylinder units that are already running, the 1505 can also trigger if the engine stalls or loses the RPM signal mid-operation. Either way, the root cause is a broken link in the starting or ignition chain.

Common Causes

Dead or Weak Battery

A discharged or failing battery is the most common culprit. If the battery doesn’t have enough voltage to turn the starter motor, the engine won’t crank, and the controller won’t see any RPM signal. This is especially true in cold weather, when battery capacity drops significantly.

Bad Starter Motor or Starter Wiring

If the battery is good but the starter doesn’t engage or turn the engine, the problem is either the starter itself or the wiring connecting the battery to the starter. Corroded terminals, loose connections, or a failed starter solenoid will prevent the engine from turning over.

Ignition Coil Failure

The RPM sensor signal comes from the ignition coil. If the coil is damaged, cracked, or has failed internally, the controller won’t receive the RPM feedback it needs, even if the engine is technically cranking. A bad ignition coil can also prevent the engine from firing once it does turn over.

Fuel System Issues (Single-Cylinder Units)

If your unit is already running and suddenly stalls with a 1505 alarm, check your fuel supply. An empty propane tank, clogged fuel line, or empty gasoline tank will cause the engine to quit and trigger the alarm when the RPM signal drops.

Engine Overload or Stall

A sudden electrical overload can cause the engine to stall so quickly that the controller registers it as an RPM loss. This is more common on single-cylinder models running near their rated capacity.

Diagnostic Checklist

Check the battery voltage. Use a multimeter to measure the battery terminals. A healthy 12V battery should read between 12.6 and 13.8 volts at rest. If it reads below 12 volts, the battery is weak or dead. Charge it fully and try starting again.
Inspect battery terminals and cables. Look at the positive and negative battery terminals for corrosion (white, blue, or green crusty buildup). Clean any corrosion with a wire brush and baking soda solution. Make sure both cable connections are tight—wiggle them by hand to confirm they’re snug.
Check the fuel supply. For propane units, verify the tank valve is open and the tank has fuel. For gasoline models, ensure the fuel tank is filled and the fuel shut-off valve (if present) is in the open position. Listen for fuel pump operation when you turn on the key.
Look for loose or damaged wiring. Trace the wires from the battery to the starter and from the ignition coil to the controller. Look for cuts, burns, or disconnected connectors. Gently reseat any loose connectors by unplugging and replugging them.
Test the starter manually (if safe to do so). With the battery fully charged and terminals clean, turn the key to the start position and listen for a clicking sound from the starter solenoid. A rapid clicking usually means low battery voltage or a bad connection. No sound at all suggests a failed solenoid or starter motor.
Inspect the ignition coil visually. Remove the spark plug wire and look at the coil for cracks, burn marks, or obvious damage. A cracked or charred coil should be replaced. If it looks intact, you may need a multimeter to test its resistance (consult your manual for spec values).
Clear the alarm and try again. Once you’ve addressed the most likely cause, clear the error code using your generator’s control panel (usually a reset button or menu option), then attempt a manual start. If the alarm returns immediately, move to the next step or call a technician.

When to Call a Pro

Stop troubleshooting and contact a Generac-certified technician if:

The battery is fully charged and terminals are clean, but the starter still won’t engage or turns very slowly.
The ignition coil shows visible cracks, burn marks, or charring.
You hear a rapid clicking from the starter solenoid even after charging the battery and cleaning connections.
The engine cranks normally but still won’t fire (no spark or fuel reaching the cylinder).
The 1505 alarm returns repeatedly after you’ve replaced the battery or cleared the code.
You’re uncomfortable working with electrical components or high-voltage systems.

Parts You May Need

12V Battery (appropriate amp-hour rating for your model)
Spark Plug (correct heat range and gap for your engine)
Ignition Coil (OEM or equivalent)
Starter Motor (if solenoid tests fail)
Battery Cable or Terminals (if corroded beyond cleaning)
Fuel Filter (if fuel system is suspect)

Frequently Asked Questions

Q: Can I start my generator manually if the 1505 alarm keeps coming back?

A: Manual pull-cord starting (if your model has one) may bypass the electronic start circuit, but the 1505 alarm indicates a real problem—either the battery, starter, or ignition system is failing. Relying on manual starts is a temporary workaround, not a fix. You should diagnose and repair the root cause to avoid being stranded without power when you need it most.

Q: Why does my generator show 1505 only in cold weather?

A: Cold temperatures reduce battery capacity and increase engine cranking resistance. A battery that works fine in summer may not deliver enough current in winter to turn the starter fast enough for the engine to fire. If the 1505 only appears when it’s cold, your battery is likely aging and needs replacement. Also, ensure your oil viscosity is appropriate for the temperature range (check your manual).

Q: Does a 1505 alarm mean my engine is damaged?

A: Not necessarily. The 1505 is an RPM sensor alarm, not an engine damage code. It means the starting or ignition system isn’t working, not that the engine itself is broken. Once you fix the battery, starter, or ignition coil, the engine should run normally. However, if the engine was running and suddenly stalled due to overload, there’s a small risk of internal damage—a technician can perform a compression test to rule that out.

Q: Can I replace the ignition coil myself?

A: On most Generac Evolution models, the ignition coil is accessible and replacement is straightforward for someone with basic mechanical skills. However, you must disconnect the battery first and follow your manual’s wiring diagram carefully. If you’re unsure about wire routing or connector types, have a technician do it to avoid creating additional electrical problems.

Disclaimer

This article provides general troubleshooting information for the Generac 1505 RPM Sensor Loss alarm. It is not a substitute for your generator’s official manual or professional service. Always consult the manufacturer’s documentation for your specific model before attempting repairs. If you are unsure about any step or lack the proper tools, contact a qualified Generac technician. Improper repair or maintenance can result in equipment damage, personal injury, or failure to provide backup power when needed.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.