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Common Onan P4500i Generator Problems and How to Fix Them

As a Cummin Onan P4500i generator user, I appreciate its reliability. However, my journey with this generator has not been without challenges. In this troubleshooting guide, I will share my personal experience with Onan P4500i generator problems and how I fixed them myself.

I have been using the P4500i generator since December 2021, and I am very happy with its performance. I can fix the most common problems by myself.

Whether you are a new or experienced user of this generator, this will help you maintain its efficiency.

Source: https://www.cummins.com/

The Onan p4500i Generator Is Not Starting

The starting issue in the P4500i generator can be caused by multiple reasons. Let’s address them one by one.

1. Battery switched off: Check the battery switch-off position. The battery can prevent your generator from starting.

Solution: Before starting the generator, always ensure the battery switch is in position.

2. No fuel: This is also a possible reason for the generator not starting.

Solution: Make sure you have enough fuel in the fuel tank.

3. Old or lousy gasoline: Gasoline fuel can also create problems when the generator starts.

Solution: Ensure that the fuel in the tank is clean and usable.

4. Clogged or dirty air filter: The air filter can also be problematic in the generator starting process.

Solution: Ensure the air filter is clean and airy; if not, clean it before starting the generator.

5. Low oil in the engine: It is also possible that a generator is stopped due to a lack of engine oil.

Solution: Check the low oil indication. If it blinks or is on, it means that you need to purr the oil in your generator.

*Always position your battery switch off before adding engine oil.

6. Spark Plug: Any fault in the spark plug can also prevent the generator from starting. So check if the spark plug is wet, broken, dirty, or improperly gapped.

Solution 1: For a wet spark plug, you need to switch off the battery switch and pull the recoil handle repeatedly a few times. This process will dry the spark plug. Then, switch the battery switch on and try to restart the generator.

Solution 2: You must replace a broken spark plug.

Solution 3: If you find the plug dirty after removing it, simply remove the dirt and place it back.

Solution 4: Properly adjust the spark plug gap (0.024-0.032 in/0.60-0.80 mm) or simply replace it with a new one.

7. Battery Drained: Sometimes, the Onan P4500i battery drains for some reason. In this case, the generator will not start without the recoil starter.

Solution: Use a recoil starter to start, refill, and recharge the battery, or you may need to replace the battery.

8. The choke is not switching auto: The choke may not switch auto on and off due to a weak or disconnected battery.

Solution: Adjust the choke manually and connect, recharge, or replace the battery.

9. Dirty spark arrester: Dirty and blocked spark arrest can also cause starting problems in generators.

Solution: Check and clean the spark arrester.

10. Cranks but won’t start: When you try to start the Onan P4500i generator, it only cranks but won’t start. In this case, you need to check if the spark plug is sparking.

Solution: Try another spark plug if the generator’s spark plug is not producing sparks. If it is still not working, it means that the light indicator of the P4500i generator is faulty and needs to be replaced.

You may be interested in Generator overload while nothing plugged in.

Onan p4500i Is Not Producing Output Power

No output problem can be faced in two possible types: first, the generator is not powering electric equipment, and second, the generator is not producing AC output.

For the first one, if your generator is not powering electric equipment, check the generator’s connection and double-check the connection cable and breakers.

The second one can accrue due to multiple causes. Let’s discuss them step by step.

1. Excited load or Faulty breaker: Check the AC circuit breaker to see if it has tripped.

Solution: Check the load on your generator. If it is over the limit, reduce the load and reset the breaker.

2. Power cord connection: The load’s power cord is not connected correctly to the generator plug.

Solution: Check and properly connect the power cord to the generator, check for faulty components in the connection, and replace them if necessary.

3. Faulty or short-circuit appliances: Faulty or short-circuit appliances can also cause power disconnection and breaker tripping.

Solution: Disconnect the connected appliance, check the appliance is in good condition on the generator, and repair the defective appliance.

*A faulty power cord can also be defective.

P4500i Overload Light Is On While Nothing Is Plugged In

When an Onan P4500i generator is running without load and shows an overload light, this can be caused by multiple reasons. This is the most frustrating problem because your generator will shut down even if you don’t load it.

1. Wiring problem: This problem can be caused by a wiring fault. Your generator’s wiring may have a short circuit.

Solution: Carefully uncover the generator and check the wiring for burnt or improper connections. Replace the faulty wiring and restart the generator.

2. Oil sensor fault: The overload light will come on even if nothing is plugged in. The generator will keep running for 10-20 minutes and then shut down.

Solution: Check the oil sensor. This problem can be caused by a faulty oil sensor. Replace the oil sensor with a new one. You can also direct the oil sensor to skip the sensing, but this is not recommended.

Recommended Maintenance Schedule for Onan p4500i

Regular maintenance can improve the performance and increase the life of the Onan p4500i. The user must follow Cummin Onan’s maintenance schedule (in the user manual). I will provide the link in this article if you still need it. Regular or hourly maintenance is initial for each generator, even without issues. What needs to be checked? Follow the instructions below.

  1. Checking of fuel and engine oil is vital before each use.
  2. Changing engine oil is essential after every 25 hours of use or the first month (First time).
  3. Cleaning the air filter and changing the engine oil is necessary after every 6 months or 50 hours of use.
  4. Cleaning the spark plug, cleaning the spark arrester, changing fuel filters, and adjusting of valve and clearance should be done after 100 hours of running or in 6 months.
  5. Replacement of spark plug and air filter is recommended once a year or 300 hours of running.


Before attempting a Cummin Onan p4500i repair, you must have a basic understanding of electric components/generators. Working with a generator with knowledge can be safe. It is always best to call a professional technician if you need more clarification on any aspect of the generator’s operation or repair.

Should you encounter any Onan p4500i generator problems or with your other generator, please do not hesitate to contact us via our contact form or by emailing us at whatgenerators@gmail.com or jasonwallance50@skiff.com.

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