Generac Error Code 1505: RPM Sense Loss
1505 means the controller stopped seeing engine RPM while it was cranking. Generac's diagnostic manual ties 1505 to twin-cylinder engines (the 13–26 kW units) and lists two causes: a starter motor problem, or a missing ignition pulse from one of the two coils. The owner's manual gives you exactly one thing to do before calling: clear the alarm, open the BATTERY menu, and if it says CHECK BATTERY, replace the battery. If it says GOOD, contact a dealer. In practice a weak or corroded battery connection is behind most of these, which is why the fix is often a $150 Group 26R and a wire brush.
What code 1505 means
Generac's support article covers the 1500 family together: "The controller detects missing or lost RPM signal during startup or while running." The Mobile Link message for 1500–1521 is "The generator has experienced a no crank RPM Sensor alarm. Please contact a dealer." On the display you'll see RPM SENSE LOSS alternating with the number.
Generac's diagnostic manual doesn't describe a separate speed sensor on the air-cooled units; every cause it lists for the 1500 family is a starter problem or a missing ignition pulse from one of the coils. Read that way, "RPM sense loss" during cranking means the starter turned the engine (or tried to) and the controller didn't see ignition at a rate it recognized. The diagnostic manual splits the family four ways:
| 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 primary coil) | Unit will not run | Check, then call |
| 1505 | RPM SENSE LOSS | Twin-cylinder, cranking: starter motor issue, missing ignition pulse (loss of one primary coil) | 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 | Unit will not run | Check, then call |
| 1515 | RPM SENSE LOSS | Single-cylinder, cranking: starter motor and/or engine issue, loss of ignition pulse | Unit will not run | Check, then call |
So 1505 specifically = a 13–26 kW unit that failed while cranking. If yours is a single-cylinder unit (the smaller air-cooled models), you'd see 1515 for the same event.
On Nexus-era units the display reads "RPM sensor failure." That manual gives the timing: during cranking the board wants a valid RPM signal within three seconds; while running, losing it for one second shuts the engine down. Evolution manuals don't publish these numbers.
Reset procedure (as the manual states it)
The owner's manual entry for RPM SENSE LOSS during cranking:
Clear alarm. Using control panel, check battery by navigating to BATTERY MENU option from MAIN MENU. If battery condition displays GOOD, contact an IASD. If control panel displays CHECK BATTERY, replace battery.
And for the running variant (the unit was running, shut down, tried to restart):
Clear alarm and remove household loads from generator. Put into AUTO and restart. If generator does not start, contact an IASD.
To clear: shutdown switch(es) to I, press OFF, ENTER at the prompt, AUTO, then MANUAL to test. Nexus: rocker OFF, ENTER twice, rocker AUTO.
Step-by-step diagnosis
- Battery menu. MAIN MENU → BATTERY. This is Generac's first and only owner-level step for 1505.
- CHECK BATTERY → replace it. Spec: Group 26R wet cell, 540 CCA minimum, or Group 35 AGM, 650 CCA (part 0H3421S). Skip to step 6 after installing.
- GOOD → keep going; a battery that reads fine at rest can still fail to deliver cranking amps through corroded terminals.
- Terminals and cables. With the unit OFF, look at both battery posts and the ground strap to the frame. White or green crust, a loose nut, or a cable that's stiff and hot near the terminal will pass a voltage test and fail under a 16-second crank. Clean with a terminal brush, retighten, add dielectric grease.
- Charger. The battery is kept up by a charger fed from the house. Check the alarm log for 2770 (charger warning) or 2780 (charger missing AC); a tripped breaker on that circuit lets the battery run down slowly over weeks and then fail during a real outage. Check the load center for the generator's utility feed breaker.
- Fuel and air — the support article's second tier. Fuel valve on, correct fuel selected, propane above 30 %, air filter clear. Generac lists these for the whole 1500 family; they matter more for 1501 (running) than 1505, but an engine that fires weakly on a starved mixture gives a ragged pulse train the controller may not count.
- Listen to it crank. Press MANUAL and stand by the unit.
- A slow, laboring crank on a known-good battery → starter motor or a mechanical problem (dealer).
- A normal crank with no firing on either cylinder → could be both coils, or fuel — go back to 1100 checks.
- Fires on one cylinder only (rough, half-speed sound) → a dead primary coil, which is precisely the diagnostic manual's second cause. Dealer, with the model number.
- Clear and test under load. If it runs clean in MANUAL, put it in AUTO and either wait for the weekly exercise or trip the utility breaker to force a transfer. A 1505 that recurs only on automatic starts is nearly always a battery that reads fine at rest.
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; 20–24 kW spec sheet |
| Spark plug, RC12YC equivalent | 0G0767A | 14–26 kW (two) | Owner's manuals |
| Air filter | 0J8478S | 14–26 kW | Owner's manual A0001846499 |
| Control panel fuse | 0D7178T | 10–24 kW | Owner's manual A0000973303 |
Ignition coils and starter motors are engine-specific (992 cc vs 999 cc) and no single part number could be confirmed against a Generac or dealer listing, so none is given. The dealer will pull it from your serial number.
Repair or replace?
Battery and terminals resolve the large majority of 1505s on units under ten years old, for well under $200. A starter motor or a coil is a few hundred dollars installed. The one expensive path is a starter that's slow because the engine is tight — a unit that seized after running low on oil — and that's a rebuild decision, not a 1505 decision. If your generator is a 2008–2012 Nexus unit with a controller Generac no longer supports and it's throwing RPM faults plus other alarms, price a new 14 kW against the repair estimate before you say yes to anything over $800.
When to call a dealer
- BATTERY menu says GOOD, terminals are clean, and 1505 comes back.
- The unit runs on one cylinder or sounds rough after it starts.
- Cranking is slow with a new, fully charged battery.
- 1505 appears with 1100 (overcrank) or 2770/2780 (charger) in the log.
- Generac support: 1-888-436-3722; warranty work through an Independent Authorized Service Dealer.
Parts and supplies mentioned on this page
A Group 26R with at least 540 CCA is the spec; the Generac-branded one is 0H3421S, but any battery meeting the group and CCA works.
- Generac 0H3421S Group 26R battery (540 CCA)Check price on Amazon
- Battery terminal brush and dielectric greaseCheck price on Amazon
- Generac spark plug 0G0767A (14–26 kW, set of 2)Check price on Amazon
Related codes and symptoms
1100 Overcrank and 1505 are cousins: both show up when a tired battery can't carry the crank, and it's common to see one, then the other. 1501 is the same fault while running and points harder at fuel. If the unit won't crank at all and shows nothing, check the control panel fuse (0D7178T) and the shutdown switches (2800).
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
My battery is only two years old. Can it really cause a 1505?
Yes. Standby generator batteries live outdoors, get trickle-charged constantly, and are asked for a full 16-second crank a few times a year. Sulfation and a corroded terminal are common well inside two years. The BATTERY menu reading GOOD is a voltage check; a load test at an auto parts store is the real answer.
What's the difference between 1505 and 1515?
Same fault, different engine. 1505 is the twin-cylinder (13–26 kW) code; 1515 is the single-cylinder code for the smaller units. The checks are identical.
Does 1505 mean the RPM sensor needs replacing?
Generac's diagnostic manual doesn't list a separate RPM sensor part for air-cooled units. Its causes for 1505 are the starter motor and a missing ignition pulse from one coil, which is what a dealer will test.
The generator ran for 20 minutes and then threw an RPM code. Same thing?
That's 1501 (twin) or 1511 (single) — RPM lost while running. The diagnostic manual points at an air pocket in the fuel line, dirty fuel, or a coil dropping out when hot. Reduce the load, clear, restart; if it repeats, it's a dealer visit.
Will Mobile Link show me 1505?
Yes — the status history shows the code, and the alert text reads "The generator has experienced a no crank RPM Sensor alarm. Please contact a dealer."