What Is Fuel Trim?
Fuel trim is the correction the engine computer applies to base fuel delivery, expressed as a percentage. Positive trim means it's adding fuel (the engine is running lean); negative trim means it's removing fuel (running rich). Short-term trim reacts instantly; long-term trim is the learned average.
In plain English
Fuel trims tell you what the computer thinks of the mixture. Trims near zero mean everything is healthy. Large positive numbers mean the engine runs lean — often a vacuum leak or fuel delivery problem. Large negative numbers mean rich — excess fuel or overstated airflow.
The pattern matters as much as the number: high trims at idle that improve at cruise point to a vacuum leak; trims that stay lean at all speeds point toward a fuel pump, filter, or airflow-sensor issue.
Example
Related trouble codes
Run your entire auto repair shop from one platform
AI diagnostics, repair guides, estimates, invoices, scheduling, CRM, inventory, and analytics — every feature in one plan, starting at $99/month with a 3-day free trial.
Start Free Trial$99/month · 3-day free trial · Cancel anytime · 2 months free annually
Related