How House Works

Generac 1501, 1505 (1500–1521): RPM sense loss

By How House Works EditorialBased on Generac manuals and Help CenterChecked against sources Updated

Generac controller or Mobile Link

1500–1521

“RPM Sensor”

Guardian · What Error Codes Do I Need to Call a Dealer For?; What Does It Mean When My Generator Displays an alarm: RPM Sensor Loss, error codes 1500-1521?; Guardian 10–26 kW Owner's Manual, p. 34
Owner checks, then technician

Includes 1501 and 1505. The owner's manuals call it RPM SENSE LOSS (Guardian 10–26 kW Owner's Manual, p. 34). Generac's article covers 1500–1522 and says owner troubleshooting is the same for every number. For Next Generation, see 1522.

What it means

The controller lost the engine speed signal. Generac’s article says it did not see a valid RPM signal during startup, or lost it while the engine was running, and lists a weak or bad battery, the fuel system, an overload, a sudden load change and missed maintenance as possible causes (Generac Help Center, “What Does It Mean When My Generator Displays an alarm: RPM Sensor Loss, error codes 1500-1521?”, Oct 17, 2025). The article covers 1500 through 1522 and says owner troubleshooting is the same for every number, so 1501 and 1505 get the same steps.

The owner’s manuals call the alarm RPM SENSE LOSS and give two cases: the unit was running, shut down and tries to restart; or it will not start in AUTO when utility power is lost Guardian 10–26 kW manual, p. 34; Next Generation 10–28 kW Owner’s Manual, p. 37.

Check these first

If it shut down while running:

  1. Clear the alarm, remove household loads from the generator, put the unit in AUTO and let it restart. If it does not start, contact an IASD, Generac’s Independent Authorized Service Dealer Guardian 10–26 kW manual, p. 34. In an outage, the manual’s restart order is: with the utility MLCB and the generator MLCB (main line circuit breaker) set to OFF, press AUTO, let the generator warm up for a few minutes, set the generator MLCB to ON, then the utility MLCB Guardian 10–26 kW manual, p. 21. Generac’s RPM article says never to start a generator under load.
  2. If you have a Generac Load Manager or Smart A/C module, check that it works. The RPM article says several large loads starting at once, or a sudden large load change, can cause this shutdown, and suggests running the unit with no load to see whether the problem is load related.

If it will not start in AUTO:

  1. Check the battery. Guardian: clear the alarm and open BATTERY MENU from the MAIN MENU; if it shows GOOD, contact an IASD, and if it shows CHECK BATTERY, replace the battery Guardian 10–26 kW manual, p. 34. Next Generation: check battery condition in the Mobile Link app and contact an IASD if it shows good Next Generation 10–28 kW Owner’s Manual, p. 37; the manual also names the Field Pro app, which Generac’s Help Center lists as “for Generac dealers only” (Generac Help Center, “What is the difference between maintenance reminders, warnings, and alarms?”, Jan 30, 2026). The RPM article puts average battery life at 3 to 5 years.
  2. Check that the fuel shutoff valve is ON (RPM article).
  3. On propane, check the tank: the RPM article says fuel pressure may be affected when the level drops below 30%. On natural gas, check your gas supplier’s outage alerts. If you smell gas, follow Generac’s gas leak steps.
  4. On a new installation, the RPM article says an undercharged battery is one of the most frequent causes, and that even a new battery should complete a full 24-hour charge cycle before use.

Then clear the alarm for your series and start the unit in MANUAL with no load. Generac’s test sets the generator MLCB to OFF, runs the unit in MANUAL for 3–5 minutes while you watch for the code, then OFF, MLCB ON and AUTO (Generac Help Center, “How do I test my generator in manual mode after clearing an error?”, Jun 4, 2026).

Replacing or cleaning the battery means working inside the enclosure. The manual’s battery inspection removes the 7.5 A fuse, the front panel and the intake side panel and disconnects the charger cable, and it calls for rubber gloves, eye protection and insulated tools Guardian 10–26 kW manual, p. 28. Generac recommends that an IASD do all maintenance Guardian 10–26 kW manual, p. 22.

When to call a licensed technician

  • The code returns after these checks, or the battery shows GOOD Guardian 10–26 kW manual, p. 34.
  • Fuel pressure and fuel quality. The RPM article says these checks need specialized equipment and must be done by a professional.
  • Fuel selection and fuel jet position, which are set at installation; on Next Generation units the installer sets the fuel in the Field Pro app (RPM article).
  • The air filter, which the RPM article lists among its maintenance checks, including ice in the filter in freezing weather. Generac recommends that an IASD do all maintenance Guardian 10–26 kW manual, p. 22.
  • When you call, Generac asks you to leave the error code on the unit, keep maintenance records and note what was happening when the alarm appeared (RPM article).

Notes for different models

  • Generac’s Guardian table lists RPM Sensor as 1500–1521. Its Next Generation table lists only 1522, with the Mobile Link message “The generator has experienced a no crank RPM Sensor alarm. Please contact a dealer.” (Generac Help Center, “What Error Codes Do I Need to Call a Dealer For?”, Sep 21, 2026).

Related Generac codes

  • 1522 “RPM Sensor”
  • 2750 “Low Battery”
  • 2100 “Overload Remove Load”

See also

All Generac codes

Generac is a trademark of its owner. How House Works is not affiliated with or endorsed by Generac.

Sources

  1. Generac Power Systems, Inc.. Guardian 10–26 kW Owner's Manual. Part No. A0001846499 Rev. C, 05/12/2023; cover title: Owner's Manual, 60 Hz Air-Cooled Generators, 10 kW to 26 kW; Evolution 2.0 / Sync 3.0 menu; printed page numbers (PDF page minus 3). Accessed Oct 13, 2026.
  2. Generac Power Systems, Inc.. Next Generation 10–28 kW Owner's Manual. Part No. A0004332577 Rev. C, 04/24/2026; cover title: Owner's Manual, 60 Hz Air-Cooled Generators, 10 kW to 28 kW; Power Zone 200 controller; printed and PDF page numbers match. Accessed Oct 13, 2026.
  3. Generac Help Center (Generac Power Systems, Inc.). What Error Codes Do I Need to Call a Dealer For?. Web article dated Sep 21, 2026 on the page; holds Generac's Next Generation and Guardian code tables. Accessed Oct 13, 2026.
  4. Generac Help Center (Generac Power Systems, Inc.). How do I test my generator in manual mode after clearing an error?. Web article dated Jun 4, 2026 on the page. Accessed Oct 13, 2026.
  5. Generac Help Center (Generac Power Systems, Inc.). What is the difference between maintenance reminders, warnings, and alarms?. Web article dated Jan 30, 2026 on the page. Accessed Oct 13, 2026.
  6. Generac Help Center (Generac Power Systems, Inc.). What Does It Mean When My Generator Displays an alarm: RPM Sensor Loss, error codes 1500-1521?. Web article dated Oct 17, 2025 on the page. Accessed Oct 13, 2026.