How to Read Fuel Trims

Specialized tuning software installed on laptops can be used to view and alter fuel trim data.

Fuel trims are the calculated difference from the factory setting of the fuel-to-air ratio on electronic fuel-injected engines. Measured as a percentage above or below the factory setting, a positive fuel-trim measurement is a higher fuel-to-air ratio, whereas a negative fuel-trim ratio is a lower fuel-to-air ratio. Fuel-to-air ratio data is stored in the vehicle's onboard computer. The computer measures the incoming air flow and injects the fuel accordingly. Increasing or decreasing the fuel trim might improve fuel economy or power output.

Things You'll Need

• OBD Scan Tool

Instructions

1. Connect the scan tool to the vehicle's onboard computer. Attach the scan tool's OBD port reader to the OBD port, located under the driver's side dash.

2. Turn on the scan tool, and allow the tool to boot to the ready state.

3. Start the engine on the vehicle.

4. Access the scan tool's menu to view the fuel trim measurements reported by the vehicle's on-board computer. The exact steps required depend on the manufacturer and model of the scan tool. Consult the owner's manual for specific instructions.

5. Drive the vehicle, and monitor the scan tool for fuel trim adjustments at different throttle positions. Typical throttle positions include idle, one-third, one-half and wide-open throttle. Fuel trim data might be different in each throttle position.

Tips & Warnings

• Fuel trim adjustment requires access to the vehicle's on-board computer using specialized tuners and software.

• Incorrect fuel-trim adjustment might result in engine damage.

References

• Automotive Service Association: Taking Your Scan Tool To The Next Level
• OBD Key: What is OBD?