Generac Error Code 1501: RPM Sense Loss While Running
Code 1501 means the generator was running and the controller lost the engine's speed signal. Generac's diagnostic manual: "Twin Cylinder + Running — twin cylinder running faults to RPM Sensor Loss. Possible causes: air pocket in fuel line, dirty fuel, missing ignition pulse (loss of one of the primary coils)." 1511 is the same fault on a single-cylinder engine; 1515 is a single-cylinder unit that lost the signal while cranking. (Twin-cylinder cranking is 1505.) Generac's owner steps for a shut-down-while-running: look for an overload or sudden load change, shed non-essential loads, check fuel valve, selection and LP level, check the air filter, clear the alarm (OFF → ENTER → AUTO), restart with no load. If it repeats, the dealer's next step is the diagnostic manual's fuel and ignition tests.
What code 1501 means
Generac's own cause list — "missing ignition pulse (loss of one of the primary coils)" — tells you how the controller senses speed: from the ignition pulses, one coil per cylinder on a twin. When those pulses stop arriving while the engine is supposed to be running, the controller has no way to know whether the engine stalled, a coil died, or a wire fell off. It logs "RPM sense loss" and shuts the unit down.
Generac's support article describes the running version in plain terms: the controller "has lost a valid RPM signal while running," with the listed causes "weak or bad battery, fuel system, overload condition, sudden load change, lack of routine maintenance." The diagnostic manual narrows the twin-cylinder running case to three: an air pocket in the fuel line, dirty fuel, or a missing ignition pulse from one of the two primary coils. Its action column sends a technician to Test 50 and Test 64 of the diagnostic manual.
That ordering matters. A twin-cylinder engine can keep running on one cylinder — roughly, badly, and with a fraction of its power — while the controller, missing one coil's pulses, calls it RPM sense loss. So "the engine was still running when it shut itself off" is a real observation, and it points at ignition rather than fuel.
| Code | Display | Meaning (Generac wording) | Severity | DIY? |
|---|---|---|---|---|
| 1501 | RPM SENSE LOSS | Twin-cylinder, running: "air pocket in fuel line, dirty fuel, missing ignition pulse (loss of one of the primary coils)" — Test 50 and Test 64 | Unit will not run | Check, then call |
| 1505 | RPM SENSE LOSS | Twin-cylinder, cranking — see the 1505 page | Unit will not run | Check, then call |
| 1511 | RPM SENSE LOSS | Single-cylinder, running: "air pocket in fuel line, dirty fuel, loss of ignition pulse" — Test 50 and Test 64 | Unit will not run | Check, then call |
| 1515 | RPM SENSE LOSS | Single-cylinder, cranking: "starter motor and/or engine issue, loss of ignition pulse" — if it cranks, Test 64; if not, Problem 15 | Unit will not run | Check, then call |
| 1518 | RPM SENSE LOSS | Not defined in any Generac air-cooled document; reported by owners on 30–38 kW liquid-cooled units. Treat as an RPM-sense variant and call a dealer | Unit will not run | Licensed pro / dealer |
| 1500, 1519–1521 | RPM SENSE LOSS | Named in the range on Generac's support index; no per-code definitions published for air-cooled units | Unit will not run | Licensed pro / dealer |
The Mobile Link message for the whole family is "The generator has experienced a no crank RPM Sensor alarm. Please contact a dealer." — the same text whether the fault occurred cranking or running, which is why the number on the controller matters more than the app message.
Reset procedure (as the manual states it)
The owner's manual's second RPM SENSE LOSS row — "unit was running and shut down" — says: clear alarm, restart, contact an IASD if it persists.
- Turn off the breakers for the big loads that were running (AC, well pump, dryer, water heater).
- Press the red OFF button.
- Press ENTER as prompted.
- Press the green AUTO button. Next Generation (Power Zone 200): hold OFF three seconds.
- Press the blue MANUAL button and let the unit run unloaded. Listen — a twin on one cylinder sounds uneven and hunts.
- In an outage, follow Generac's restart article: generator OFF, MLCB open, house loads off, AUTO, then MLCB on and loads back one at a time. "Starting your generator with all loads applied can damage the generator."
If it shuts down again, Generac's answer is a dealer, not a third reset.
Step-by-step diagnosis
- What was happening when it stopped? A sudden big load — compressor start, well pump — dragging the engine down until the pulses fall out of window is a common 1501 on a unit near its limit. Run the loads through the wattage calculator. If the peak exceeds the rating, this is sizing, and a load-shed module or a soft-start on the AC is the fix. Generac's article lists assessing an overload or sudden load change and verifying load management as the first steps.
- Fuel. Fuel valve fully open; fuel selection (NG/LP) matches the installation; propane above 30 %. An "air pocket in the fuel line" in Generac's wording is what you get after a tank refill, a regulator swap, or a gas-line job that let air in — the engine runs, then leans out and stumbles. Running it in MANUAL for ten minutes unloaded usually purges it.
- Dirty fuel. On propane, that's a contaminated tank or a regulator passing oil; on natural gas it's rare. Either way it's the gas contractor's problem, not the generator's.
- Air filter. Clogged or iced (0J8478S on 14–26 kW). A choked intake makes an engine stumble under load exactly like lean fuel.
- Battery. Generac lists a weak battery first even for a running fault, because the controller's own supply matters. Check the BATTERY menu; above 12.6 V is what Generac's article calls acceptable. Below that, charge 24 hours and see battery replacement.
- Did it run rough before it quit? Uneven note, vibration, one exhaust port cooler than the other → a coil, plug, or plug wire on one cylinder. Spark plugs (0G0767A) are an owner job with the unit OFF and the battery disconnected; the coils and the ignition test (Test 64) are the dealer's.
- 1515 specifically. A single-cylinder unit that cranked but never produced a speed signal. Generac's split: engine cranks → ignition test; engine doesn't turn → starter/engine problem. Check battery and cables first; if the starter spins but the engine won't catch, don't keep cranking.
- Repeats with light load, clean filter and good fuel? The diagnostic manual's Test 50 and Test 64 need the dealer's instruments.
Parts, numbers, and consumables
| Part | Part number | Fits | Where verified |
|---|---|---|---|
| Battery, Group 26R, 540 CCA (or Group 35 AGM 650 CCA) | 0H3421S | 10–26 kW air-cooled | Owner's manuals A0001846499 / A0000973303 |
| Spark plug, RC12YC equivalent | 0G0767A | 14–26 kW (two per engine) | Owner's manuals |
| Air filter | 0J8478S | 14–26 kW | Owner's manual A0001846499 |
| Air filter | 0E9371AS | 10 kW | Owner's manual A0001846499 |
| Control panel fuse, 7.5 A | 0D7178T | 10–24 kW | Owner's manual A0000973303 |
Ignition coils are engine-specific (the 14–26 kW twins use different coils than the single-cylinder units) and no part number could be confirmed with fitment from a Generac listing, so none is given here.
Repair or replace?
Most 1501s cost nothing but a reset and a conversation about load. A spark plug pair is a few dollars; a coil is a modest part plus a service call. Fuel-side fixes belong to the gas contractor. None of these argue for replacing the generator. The exception is the pattern where a unit throws 1501 every time the AC starts and was undersized on the day it was installed — that's a sizing problem no repair will cure, and the choice is between a load-management add-on and a larger unit.
When to call a dealer
- 1501, 1511 or 1515 returns after loads are shed and fuel, filter and battery check out.
- The engine ran rough or on one cylinder before it stopped — ignition test.
- 1518 or any other RPM-sense number not in the table above — no owner procedure exists.
- Fuel pressure is suspect — measured under load with a manometer, not guessed.
- Generac support: 1-888-436-3722.
Parts and supplies mentioned on this page
A fresh battery and a pair of plugs at the recommended interval remove the two owner-serviceable causes on this list.
- Generac replacement battery 0H3421S (Group 26R)Check price on Amazon
- Generac spark plug 0G0767ACheck price on Amazon
Related codes and symptoms
1505 is the cranking version of this fault on twin-cylinder engines and the most-searched number in the family. A 1600 Underspeed or 2100 Overload in the same log entry confirms the load theory. If the unit never cranked at all, that's 1100 Overcrank territory — or a dead battery with no code, covered in Generac won't start.
Not sure the generator can carry the load?Generator Wattage Calculator Running watts, the largest starting surge, and a generator class from manufacturer charts.Frequently asked questions
What is the difference between Generac 1501 and 1505?
Both are RPM sense loss on twin-cylinder engines. 1501 happened while the unit was running (fuel air pocket, dirty fuel, or a lost ignition pulse); 1505 happened while cranking (starter or ignition). The number tells you when the signal was lost.
Can a twin-cylinder Generac run on one cylinder?
Yes, badly — and that is one way to get 1501. With one coil dead the controller sees half the expected pulses and shuts down. An uneven note before the shutdown points at ignition.
What does Generac code 1518 mean?
Generac has not published a definition for 1518 in its air-cooled documents; it is reported by owners on larger liquid-cooled units. Treat it as an RPM-sense alarm and call a dealer.
How do I reset a Generac 1501 alarm?
Shed loads, then OFF, ENTER, AUTO and a test in MANUAL with nothing connected. Next Generation units: hold OFF three seconds. If it comes back, Generac says to contact a dealer rather than clear it again.
Does a low battery cause 1501?
Generac lists a weak or bad battery as the first cause for the family. Check the BATTERY menu on the controller — above 12.6 V is acceptable per Generac's article; below that, charge for 24 hours or replace.