Plain English: Code 42 indicates that your Onan generator’s control processor detected a memory error during its startup self-test and cannot operate safely.
What Code 42 Means
When your Cummins Onan generator displays Code 42, it’s telling you that the microprocessor—the “brain” of the control board—found a problem with its own internal memory during the power-up self-test. Think of it like a computer checking its own hard drive at startup and discovering corrupted files. The processor is designed to shut down rather than run with faulty instructions, because operating with bad firmware could damage the engine or create a safety hazard.
This code appears on MicroLite, MicroQuiet, Marquis Gold, QG, and RV QD series generators. It’s classified as an alarm, meaning your unit will not start or will stop running until the issue is resolved.
Why This Happens
The microprocessor’s ROM (Read-Only Memory) stores the permanent instructions that tell the generator how to start, run, and protect itself. Several things can corrupt or damage this memory:
- Power surges or lightning strikes – A nearby lightning strike or severe electrical surge can damage the control board’s circuits, including the memory chip.
- Prolonged storage without use – Generators left idle for years sometimes develop memory errors, especially if stored in damp or extreme temperature conditions.
- Manufacturing defect – Rarely, a control board leaves the factory with a faulty ROM chip.
- Loose or corroded connections – Poor connections between the control board and the engine can cause intermittent power delivery to the processor, leading to memory errors.
- Water or moisture intrusion – Humidity, condensation, or water damage inside the generator enclosure can corrode the control board.
Diagnostic Checklist
Before you schedule a service call, try these steps in order. They’re listed from cheapest and easiest to more involved.
- Check the battery voltage. A weak or dead battery can cause the processor to malfunction during self-test. Using a multimeter, measure the voltage across the battery terminals. It should read at least 12 volts. If it’s below 11 volts, charge the battery fully and try starting the generator again. A low-voltage condition can trigger false alarms.
- Inspect the battery terminals and cables. Corroded or loose connections at the battery can prevent proper power delivery to the control board. Disconnect the negative terminal, clean both terminals with a wire brush, and reconnect firmly. Ensure the cable is not cracked or frayed.
- Look for visible water damage or corrosion on the control board. Open the generator’s access panel (consult your manual for the location) and visually inspect the circuit board. If you see white or green crusty deposits, rust, or water stains, moisture has entered the enclosure. This is a sign the board likely needs replacement.
- Check that all connectors are seated firmly. The control board has several connectors plugging into it from the engine sensors, battery, and alternator. Gently reseat each connector by unplugging and replugging it. Sometimes vibration or corrosion can loosen these connections enough to cause intermittent errors.
- Verify the generator is in the correct operating mode. Consult your manual to confirm the control panel switches (if any) are set to the proper position for your application. An incorrect mode setting can sometimes trigger processor errors.
- Attempt a hard reset. Some Onan generators allow you to clear error codes by disconnecting the battery for 30 seconds, then reconnecting it. This performs a full power cycle of the control board. Check your manual to see if your model supports this procedure. If the code returns immediately, the ROM error is persistent and the board needs service.
- Inspect the generator’s environment. Make sure the unit is stored in a dry location with moderate temperature. If it’s been exposed to rain, snow, or high humidity, move it to a sheltered area and allow it to dry for 24–48 hours before attempting to start it again.
When to Call a Pro
If you’ve completed the checklist above and Code 42 still appears, or if any of these conditions are present, contact an authorized Cummins Onan dealer:
- The code reappears immediately after a hard reset.
- You see visible corrosion, water stains, or physical damage on the control board.
- The battery is fully charged and all connections are tight, but the error persists.
- The generator has been struck by lightning or exposed to a severe power surge.
- You are not comfortable opening the generator enclosure or working with electrical components.
Code 42 almost always requires control board replacement or repair by a qualified technician. The ROM memory cannot be user-repaired, and attempting to operate the generator with a faulty processor can lead to engine damage or safety issues.
Parts You May Need
Depending on the root cause, a technician may need to replace or service:
- Control board / microprocessor module
- 12V battery (if weak or damaged)
- Battery cables and terminals
- Alternator (if it’s supplying incorrect voltage to the board)
- Wiring harness connectors
Frequently Asked Questions
Can I replace the control board myself?
Control board replacement is possible for experienced DIYers, but it requires careful handling, proper tools, and knowledge of your specific model’s wiring diagram. Most homeowners should have an authorized dealer perform this work to ensure proper installation and avoid voiding the warranty. If you’re confident in your skills, consult your manual for the exact procedure.
Is Code 42 covered under warranty?
If your generator is still under the manufacturer’s warranty and the ROM error is due to a defect (not user damage or environmental exposure), Cummins Onan will typically cover the repair. Contact an authorized dealer with your model number and purchase date to check your coverage.
Will Code 42 go away on its own?
No. A ROM memory error is permanent until the control board is repaired or replaced. The processor will continue to fail its self-test every time you try to start the generator. Ignoring the code will not resolve it, and attempting to bypass the safety shutdown can damage the engine.
What’s the difference between Code 42 and other Onan error codes?
Code 42 specifically indicates a problem with the processor’s internal memory, not with engine sensors, fuel, or electrical output. Other codes (such as those for low oil pressure, high temperature, or low voltage) point to different system failures. Always refer to your manual to understand what each code means for your model.
Final Notes
Code 42 is a serious alarm that requires professional service. The good news is that once the control board is replaced or repaired by an authorized Onan dealer, your generator should return to normal operation. In the meantime, do not attempt to force the generator to run or bypass the safety shutdown.
Disclaimer: This article provides general information about Code 42 on Cummins Onan generators. It is not a substitute for your generator’s official manual or the guidance of an authorized dealer. Always consult your manufacturer’s documentation and follow all safety procedures for your specific model before attempting any repairs or maintenance. Improper service can result in equipment damage, personal injury, or voiding of your warranty.
Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.