Fuel Level Switch Indicates Empty or Low Fuel
What does ONAN-41 mean?
The ONAN-41 (Cummins Onan) diesel fault code means: Fuel Level Switch Indicates Empty or Low Fuel. This is a moderate severity code.
Common Symptoms
- Genset displays fault code 41 or flashes LED 4 times then 1 time on the control panel
- Genset cranks but will not start, or shuts down shortly after starting
- RED fault light illuminates on the genset panel
- Remote panel shows a fault or alarm condition with no RUN status
- Fuel gauge on the RV or vehicle dashboard reads near empty
- Genset that was running shuts itself down unexpectedly during operation
- Genset passes crank sequence but dies before reaching governed speed
Probable Causes (Ranked by Likelihood)
- Fuel tank is actually empty or below the genset pickup tube level Very Likely
- Faulty or stuck fuel level float switch sending a false empty signal Likely
- Corroded or broken wiring between the fuel level switch and the genset controller Likely
- Fuel level switch connector loose or disconnected at the tank or controller harness Possible
- Diesel pickup tube inside the tank is positioned too high, leaving usable fuel unreachable Possible
- Air in the fuel line after running dry prevents restart even after refueling Possible
- Controller internal fault misreading a valid fuel level signal Less Likely
Step-by-Step Diagnostic Procedure
Check the actual fuel level in the tank first. On most RV diesel gensets, the genset draws from the main vehicle fuel tank and shares the fuel level sender. If the tank is below roughly one quarter, the genset pickup may not reach fuel even when the gauge shows some remaining. Fill the tank to at least half before continuing diagnosis.
After refueling, clear the fault by pressing and holding the STOP button for a few seconds, then attempt a fresh start. Some controllers require a manual reset before they will retry.
Inspect the fuel level switch wiring harness. Trace the two-wire circuit from the fuel tank sender to the genset controller. Look for chafed insulation, corroded terminals, or connectors that have vibrated loose. Wiggle the harness while watching the fault indicator. If the fault clears or comes and goes, you have a wiring problem.
Measure resistance across the fuel level switch terminals with a multimeter. A typical float switch reads near 0 ohms when the float is up (tank full) and open circuit or high resistance when the float is down (tank empty). Compare your reading to the spec in your genset service manual. A switch that reads open with a full tank is faulty.
If the tank has fuel and the switch tests good, check for air in the fuel line. On diesel models such as the Onan QD 6000, QD 8000, and QD 10000, running the tank dry introduces air into the fuel system. You must prime the system by cranking in short 10-second bursts with 30-second rests, or by using the manual priming pump if your model has one. Attempting long cranks will wear the starter.
Check the voltage at the fuel level switch connector with the key switch in the RUN position. You should see battery voltage on the supply wire (typically around 12 VDC). No voltage here points to a blown fuse or broken wire upstream of the switch.
If all wiring and switch resistance checks pass and the tank is full, the fault is likely inside the controller. At this point you need Onan InPower diagnostic software or an Onan service display panel to read live sensor data. Call a qualified Cummins Onan technician for further diagnosis.
Frequently Asked Questions
What does Onan code 41 mean?
Code 41 means the genset controller is reading the fuel level switch as empty or below the minimum run level. The genset will refuse to start or will shut itself down to prevent damage from running without fuel. In most RV setups, the genset shares the main vehicle fuel tank, so if you are low on diesel or gasoline, the genset is the first thing to lose fuel because its pickup tube sits higher than the vehicle engine pickup.
Can my generator still run with this code?
No. Code 41 is a hard shutdown and lockout. The controller will not allow the genset to run while this fault is active. You must clear the underlying cause, add fuel, reset the fault, and then restart before the genset will operate again.
How much does it cost to fix?
If the tank is simply empty, the fix is the cost of fuel. If the float switch has failed, a replacement switch is typically $30 to $80 in parts and you can swap it yourself in about an hour on most models. If air got into the fuel system after running dry, a shop may charge one to two hours of labor ($80 to $200) to bleed and prime the fuel system. If the issue is controller-related, diagnostic fees alone can run $100 or more before any parts.
Will the generator start the next time the power goes out?
Not until the fault is resolved. Code 41 blocks an automatic start. If your genset is connected to an ATS for backup power, it will not pick up the load during an outage while this fault is active. Fill the tank, reset the fault, and run a manual test cycle to confirm the genset starts before relying on it for emergency backup.