High Engine Coolant Temperature Shutdown (1 Blink)
The ONAN-1 (Cummins Onan) diesel fault code means: High Engine Coolant Temperature Shutdown (1 Blink). This is a critical severity code.
- Keep running?
- No — shut it down
- DIY difficulty
- easy
A $30 car code reader can't do diesel. The HD7000 reads full-system codes and does parked DPF regen, idle/speed-limit, and service resets from the cab — on everything from a 6.7 Cummins/Power Stroke/Duramax pickup to Class-8 trucks (Detroit, Paccar, CAT, Volvo, Mack, International).
Full-system 6/9/16-pin diesel scan tool for Cummins, Paccar, CAT, Detroit and more — plug-and-play, no subscription. A cheaper way to read heavy-duty codes a basic OBD2 scanner skips entirely.
Affiliate links — we earn a commission at no extra cost to you.
What Are the Symptoms of ONAN-1?
- The generator shuts down on its own while running
- The status light on the Start/Stop switch blinks once, pauses, and repeats
What Causes ONAN-1? (Most Likely First)
- The engine overheated: Cummins defines one blink as a shutdown due to high engine coolant temperature Very Likely
- Hot air not escaping the generator compartment, such as blocked inlet or outlet openings Possible
- Low coolant on a liquid-cooled diesel model Possible
Can I Keep Running It with ONAN-1?
No — shut it down. ONAN-1 is rated critical severity. Continuing to run the unit in this state risks further damage — get it looked at before putting it back in service.
Common Fixes by Vehicle
What techs usually find when diagnosing ONAN-1 on specific platforms — tap a platform for the fix and the exact part:
Onan QG / Microquiet RV gensets Easy DIY
Onan blink-1 (High Engine Coolant Temperature) on RV gensets is nearly always blocked compartment airflow. Check the genset compartment intake and exhaust louvers for blockage (mud daubers, debris, even storage items packed against the vent). Clean the cooling fins on the engine itself with compressed air. If it still trips after cleanup, the temperature switch ($25-$60) on the cylinder head is the next part. On Quiet Diesel models (Kubota-powered) check coolant level and the radiator fan belt.
Labor: 30-60 minFrequently Asked Questions
What does one blink mean on an Onan RV generator?
Cummins defines one blink as a shutdown due to high engine coolant temperature. It is not the low oil pressure code: that is two blinks.
Is it the same as code 1 on the QD 3200?
No. The RV QD 3200 does not blink codes; it shows text messages on its display, including an oil indicator light for low oil pressure or high oil temperature.
Mechanics Guide Step-by-step diagnosis
How Do You Diagnose ONAN-1?
Let the generator cool before touching it or opening the compartment.
Check that nothing blocks the air openings of the generator compartment.
On a liquid-cooled diesel model, check the coolant level only when the engine is cold.
The code stops blinking after five minutes (15 minutes on Series HGJAA). To see it again, press Stop three times within five seconds; the last logged fault blinks even if the condition has cleared.
If it shuts down again on one blink, stop restarting it and have a Cummins Onan dealer or service center check it.
Sources 1
This page is built from documented references. Verify against your own service info before repair work.
Which Vehicles Commonly Get ONAN-1?
- Cummins Onan RV generators with a status light on the Start/Stop switch (gasoline, LP and diesel models)
- Not the RV QD 3200, which shows text messages on its display panel
Explore More
Was this helpful?
Send this to whoever is fixing the truck — your tech, or the person asking what the fault means.