Generac 1505 RPM Sense Loss: What It Means & What To Do

Last updated: October 2026

Error 1505 — RPM Sensor Loss — is a cranking alarm. In Generac’s own description: the controller told the engine to crank, but got no signal back that the engine was actually turning. It triggers a shutdown every time it occurs and it will not self-clear. It is not one of the generic “low oil pressure” codes some listicles claim — those aggregators get this one wrong often enough that it’s worth saying out loud.

First, know which RPM code you have

The 1500 family splits on two axes — was the unit cranking or already running, and is the engine single- or two-cylinder:

• 1505 — 2-cylinder, cranking: no RPM signal during the start attempt.
• 1515 — 1-cylinder, cranking: same fault, smaller engine.
• 1501 — 2-cylinder, running: the unit was running and stalled or lost the signal (fuel, coils, or an overload stall lead that list).
• 1511 — 1-cylinder, running: same as 1501.

This article is about the cranking pair (1505/1515). If your unit starts and then dies with 1501/1511, the fuel branch leads — different tree.

The cause list, in the order technicians work it

Generac’s troubleshooting for this family is consistent across its documentation and its certified experts’ answers:

1. The battery — bad, dead, improperly connected, or with loose or dirty terminals. This is cause #1 by a wide margin.
2. The starter contactor — a small black relay near the controller with four wires; it converts the controller’s start signal into the higher-current signal that engages the starter solenoid. A fault or loose connection here stops the crank before it starts.
3. The starter motor — a fault, a bad connection, or the small 16-gauge wire that signals the solenoid to engage being loose or disconnected.
4. The ignition coils — defective, or damaged (they can contact the flywheel). The RPM signal itself reaches the controller from the coils on the 18-gauge wire — wire 18.
5. The engine itself — internally damaged and physically unable to turn. Listed for completeness; it’s the rare branch.

The one question that splits the tree

Before touching anything, answer this from your last start attempt: what did the engine physically do?

• Click, or nothing: the crank never happened. Battery and connections first, then the contactor branch.
• Turns over slowly, labored: the classic battery-under-load signature (next section).
• Turns over at normal speed, then stops with the alarm: the engine turned but the controller never saw it — that’s the signal path (wire 18, coils), and after ruling out the battery it’s dealer diagnosis.

Why the battery fools everyone: resting voltage lies

Generac’s own troubleshooting guidance for this code is blunt: start with the battery, and load-test it — don’t just check voltage. A bad battery will very often show normal voltage at rest, then drop toward zero the moment a load is applied. The practical thresholds technicians use: sagging below roughly 10 volts during cranking points at the battery or its connections; a battery resting below about 12.4 volts that won’t recover on charge is a replacement. Terminals matter as much as the battery itself — clean to bright metal, tight, ground side included. Generator batteries live 3–5 years, and the correct replacement for Generac air-cooled units is a 12-volt Group 26R with at least 525 CCA. The full battery picture — including the charger faults that slowly murder good batteries — is in our low battery guide.

Where the homeowner sequence ends

Battery proven good under load, terminals clean and tight, and the code still sets — the remaining causes (contactor tests, starter circuit, ignition coils, wire 18) are multimeter-and-manual work, and on a unit under warranty they’re the dealer’s work regardless: Generac-certified experts routinely warn that DIY repair beyond basic checks can void coverage. When you call, lead with the exact sentence this article gave you: “1505, cranks at normal speed” or “1505, just clicks” — it routes the technician to the right test first, which is real money on a visit that averages $273 before parts (Angi, Sept 2026).

If your code is actually 1100 — the engine cranks and cranks without the RPM complaint — that’s the fuel tree: see Generac Error Code 1100 Overcrank.

Sources: Generac Evolution e-Code reference (RPM Sensor Loss 1501/1505/1511/1515) and Generac Diagnostic Repair Manual (G0070420) via ManualsLib; Generac support reset guidance for the RPM sense loss family.