FaultCode Pro
Generac · Home standby generator
1505 Serious

RPM Sense Loss Alarm

Severity5/10

In short

Generac 1505 generally means the controller lost the RPM signal needed to confirm that the engine is turning or running correctly. The engine may not be cranking fast enough, may have started and stalled, or the speed / ignition signal may be missing. Common causes include battery and starter problems, fuel or ignition failure, damaged signal wiring, and a failed sensor or controller. The exact RPM alarm logic varies by controller and engine.

Severity
5/10
Typical pro cost
$150–$3500
Most likely cause
Weak battery, corroded cable, poor ground, or starter unable to reach cranking speed
Cheapest likely fix
Restore fuel supply or repair regulator / pressure issue · DIY $20-500

Is it safe to keep operating with 1505?

Do not keep attempting starts with 1505 active. Set the controller to OFF before inspecting the generator, keep away from the fan and belts, and treat the fuel system as a fire and explosion hazard. If there is fuel odor, smoke, oil leakage, or mechanical noise, stop and use an authorized service dealer.

Symptoms

  • 1505 or RPM Sense Loss on the controller
  • Starter cranks but engine speed is not recognized
  • Engine catches and stops quickly
  • Generator shuts down during an automatic exercise
  • Alarm appears after a weak-battery event, fuel interruption, or electrical storm

Common causes (most → least likely)

Weak battery, corroded cable, poor ground, or starter unable to reach cranking speed
Most common first area to test
$100-600
Fuel valve closed, low gas pressure, empty LP supply, or restricted fuel delivery
Common
$0-600
Worn spark plugs, ignition coil, plug lead, or ignition control fault
Common on overdue maintenance
$50-650
RPM pickup / sensor connector, harness, grounding, or signal fault
Possible and more likely when cranking sounds normal
$100-700
Controller, starter solenoid, or mechanical engine problem
Less common — verify basic engine conditions first
$300-3,500

How to diagnose it (before buying parts)

  1. 1 Set the controller to OFF and disable automatic starting. Wait for all moving parts to stop before opening the enclosure.
  2. 2 Read the complete alarm history and note whether the engine failed to crank, cranked without starting, or ran before 1505. Check for companion alarms.
  3. 3 Inspect and load-test the starting battery, clean the terminals, and check the negative cable and engine ground. A good display voltage does not prove the battery is healthy under load.
  4. 4 Verify fuel availability and the open manual shutoff. If pressure is in doubt, have a qualified gas technician test it without loosening fuel fittings.
  5. 5 Inspect spark-plug leads, ignition connectors, and accessible RPM-signal wiring for moisture, corrosion, loose locks, or rodent damage.
  6. 6 A qualified technician should test the RPM / ignition signal during cranking and running, and compare the result with the service data for the exact controller and engine.
  7. 7 After repair, clear the alarm using the model procedure and perform one supervised start. Confirm stable RPM, no abnormal noise, and normal output before returning the controller to AUTO.

Repair options & cost

Replace or charge the starting battery and repair battery connections Easy-Moderate · 30-90 min
DIY $100-400 Pro $150-600
Restore fuel supply or repair regulator / pressure issue Moderate-Hard · 1-3 hrs
DIY $20-500 Pro $200-800
Replace spark plugs, ignition part, or damaged signal harness Easy-Moderate · 30-180 min
DIY $30-500 Pro $150-800
Replace RPM sensor, starter / solenoid, or controller after testing Hard · 2-8 hrs
DIY $150-2,500 Pro $450-3,500

By model / series

Evolution 2.0 and later standby controllers

1505 is commonly presented as RPM Sense Loss. The controller may distinguish a no-start condition from a stall using related codes; read the full alarm history.

Evolution 1.0 and older Nexus controllers

RPM alarm numbers, sensor locations, and clear sequences can differ by generation. Match parts to the controller and engine serial information.

Air-cooled versus liquid-cooled units

Cranking speed and RPM signal testing are not interchangeable between engine families. Use the appropriate service manual and do not substitute generic sensor values.

Frequently asked questions

What does Generac code 1505 mean?

It generally means the controller lost the RPM signal or did not see the engine running as expected. A battery, fuel, ignition, sensor, wiring, starter, or controller fault can cause it.

Does 1505 mean the RPM sensor is bad?

Not necessarily. A slow starter, weak battery, fuel failure, or engine stall can also produce the alarm. Test the operating condition and wiring before buying a sensor.

Can I clear 1505 and try again?

Follow the manual's clear sequence for one controlled test after basic checks. If the alarm returns, stop repeated starts because they can drain the battery and create a fuel or exhaust risk.

What is the difference between Generac 1501 and 1505?

Both belong to the RPM-sense-loss family, but their exact trigger and wording depend on the controller and engine. Use the exact model alarm table rather than treating them as identical.

Related codes

Sources and model caveats

Error-code meanings and test values can vary by model, controller generation and installation. Confirm the procedure against the manual for your exact equipment.

  • Generac Support, RPM Sense Loss alarms in the 1500-series, and the owner's manual for the exact model
  • Generac controller and engine service manuals for RPM signal testing, start-system diagnosis, and fuel safety