Onan generator fault codes: reading the blinking status light

Every documented Onan fault code

Compiled from the operator manuals for the air-cooled KY and KYD sets and for the liquid-cooled HDKCA/HDKCB Quiet Diesel, with each manufacturer definition and the numbers it actually gives. Where a series column is filled in, the code exists on that family and not across the whole range.

Codes 33 and 44 are unassigned by design. Codes without a link are controller and board faults whose documented corrective action is a dealer visit; there is no owner-side procedure to add to them.

There is no single Onan fault code list

This is the trap. Cummins publishes a different table per genset family, and the differences are not cosmetic. The air-cooled gasoline and LPG sets — the KY Microlite/Emerald family behind most RV QG units — document nineteen codes and only two first-level blinks, three and four. The liquid-cooled Quiet Diesel sets document a partly different table, add four codes the gasoline sets do not have (19 actuator open or shorted, 22 actuator overload, 24 temperature sender, 57 overprime), and use one and two blinks as first-level faults.

Same number, different fault: code 36 is "Engine Stopped" on the air-cooled sets and "Mechanical Fault" on the Quiet Diesel, where it has a precise definition the gasoline manual never gives — engine speed fell below 1000 rpm for half a second without the controller asking for it. Code 32 is "low cranking speed, under 180 rpm for more than two seconds" on the gasoline sets, but on the diesel it is "starting fault: no cranking speed detected for twelve seconds". Code 31, overspeed above 3400 rpm, appears in the tables of other series and in neither of the two operator manuals above.

So before you act on a number, find out what you own. The plate on the genset carries the model and spec letter — KY, KYD, KV, HDKCA, HGJAA and so on — and the operator manual for that spec is the only table that is binding for your set.

The overcrank number Cummins never settled

Code 4 means the starter turned the engine and it did not fire. How long it cranked before giving up is stated four different ways in Cummins’ own documents: the KY operator manual says "15 to 20 seconds" in its introduction and "exceeded 20 seconds" in its troubleshooting table four pages later — inside the same PDF. The 2019 RV Generator Handbook says 30 seconds. The Quiet Diesel manual says 35 seconds.

For diagnosis it barely matters, and that is worth saying plainly: code 4 is not about the exact second count, it is about fuel, spark and cranking speed. It matters for trusting sources. If a page quotes one of these numbers as the Onan overcrank threshold without naming the series, it copied a table without checking it.

The practical version: on a shared tank the genset pickup tube sits higher than the vehicle engine pickup, so the generator runs dry while the dash gauge still shows fuel. That single fact explains more code 4 and code 36 shutdowns than every electrical cause put together.

The code you are reading may already be history

The controller keeps blinking the last logged fault even after the condition that caused it is gone. Someone tops up the fuel, restarts, sees the light still blinking 3 and 6, and starts hunting a fault that fixed itself.

Treat the code as a log entry, not a live reading. Correct the obvious cause, run the set, and see whether it shuts down again — the next real shutdown will overwrite what you are looking at.

What a dead lamp and a lamp that quits mid-crank are telling you

No light at all is not a code. Cummins lists it separately as faulty connections or no battery voltage — start at the battery terminals and the genset ground, not at the control board.

A lamp that lights, goes out during cranking and leaves the engine stopped is a third case again: an internal short has tripped a self-resetting protective device. That one is documented as a dealer job if it repeats.

Common questions