When your Cummins engine displays a warning light or check engine indicator, it’s signaling that something isn’t working as it should. These alerts correspond to Cummins fault codes — diagnostic messages that help you identify and resolve issues before they lead to costly repairs. Knowing how to read Cummins fault codes is an essential skill for truck owners, mechanics, and fleet operators.

1. What Are Cummins Fault Codes?

Cummins fault codes are numerical indicators that pinpoint specific problems in your engine’s systems.
Each fault code corresponds to a component or malfunction detected by the Electronic Control Module (ECM).

For example:

• Fault Code 111: ECM Power Supply Voltage Low

• Fault Code 415: Injector Solenoid Circuit – Cylinder 3

• Fault Code 2639: Diesel Particulate Filter Differential Pressure

These codes allow users to quickly identify the cause of performance issues or sensor failures.

2. How to Check Cummins Fault Codes Without a Scanner

If you don’t have a diagnostic tool, you can still retrieve the fault codes manually on many Cummins-powered vehicles:

1. Turn the ignition key ON and OFF three times within five seconds (do not start the engine).

2. Watch the “Check Engine” light — it will flash sequences representing the fault codes.

   • Example: two flashes, pause, then three flashes = Code 23.

3. On newer trucks, fault codes may appear directly on the instrument panel display.

This method is simple and effective for quick diagnostics on the go.

3. Using a Diagnostic Tool

For more precise results, a diagnostic scanner or Cummins INSITE software can be used.
These tools connect to your vehicle’s OBD-II or data link connector to read, interpret, and clear codes.
They provide additional details such as engine load, sensor data, and historical fault logs, giving a complete picture of your engine’s health.

4. How to Reset Cummins Fault Codes

Once the issue is fixed, you can clear the fault codes by:

• Using a diagnostic scanner, or

• Turning the ignition ON for 30 seconds and OFF for 30 seconds, repeating several times.

However, it’s always best to fix the root problem before clearing the code — otherwise, the fault may reappear.

5. Common Reasons for Fault Codes

Some frequent causes of fault codes in Cummins engines include:

• Sensor malfunctions (e.g., temperature, pressure, or boost sensors)

• Fuel injection or air intake issues

• Low voltage or ECM communication errors

• Exhaust aftertreatment system problems

Addressing these promptly helps maintain your engine’s performance and extend its lifespan.

6. Final Thoughts

Understanding how to read Cummins fault codes empowers you to identify issues early, maintain peak performance, and avoid unexpected downtime. Whether you’re using manual methods or professional diagnostic tools, reading these codes is the first step toward keeping your Cummins engine running smoothly and efficiently.