Alignment Diagnostics and Fleets
OBD Fuel Trim Calculator
Combine short- and long-term OBD fuel trims and compare banks. Interpretation depends on operating mode, load, temperature, fuel system, and manufacturer strategy.
Create a documented input set
Check labels, prefixes, suffixes, and percentage bases before calculating.
The measurement being modeled
Combine short- and long-term OBD fuel trims and compare banks — use the result to decide what to inspect next, not to skip the inspection.
Interpretation depends on operating mode, load, temperature, fuel system, and manufacturer strategy — that condition defines when average combined fuel trim is comparable with another result.
The current equation stops before the step needed to estimate camber magnitude from rim displacement measurements, which is handled by the Camber Angle from Rim Measurements.
Turning the inputs into a result
Bank 1 short-term trim. Current short-term correction for bank 1 — for this measurement, identify whether the reading is taken at the source, charger, battery, or accessory.
Document Bank 1 long-term trim as learned long-term correction for bank 1 — this means you should use a measurement or specification from the exact component and operating condition being evaluated.
The Bank 2 short-term trim entry represents current short-term correction for bank 2 — before calculating, identify whether the reading is taken at the source, charger, battery, or accessory.
Bank 2 long-term trim: Learned long-term correction for bank 2 — a compatible entry should use a measurement or specification from the exact component and operating condition being evaluated.
In “combined bank trim = short-term trim + long-term trim,” the calculation does not infer a missing vehicle measurement.
No term beyond bank 1 short-term trim, bank 1 long-term trim, bank 2 short-term trim, and bank 2 long-term trim is introduced in “combined bank trim = short-term trim + long-term trim.”
Using the supporting measurement
Average combined fuel trim answers “Combine short- and long-term OBD fuel trims and compare banks.” The additional displays, Bank 1 combined trim, Bank 2 combined trim, and Bank-to-bank difference, are a different view of the same entered measurements.
Fuel trims do not identify a failed component by themselves — when that condition changes, compare separate calculator runs instead of blending the inputs.
Because interpretation depends on operating mode, load, temperature, fuel system, and manufacturer strategy, a disagreement between average combined fuel trim and an outside reference should trigger a review of bank 1 short-term trim and bank 2 long-term trim.
For a second calculation that will calculate on-time preventive-maintenance compliance and outstanding work, use the Preventive Maintenance Compliance.
Operating conditions outside the formula
A sensor or comparison value can narrow an investigation, but it cannot identify the failed part without the specified physical tests — for bank 2 long-term trim, the page specifically expects learned long-term correction for bank 2.
If your notes raise the question of how to estimate alternator current margin after loads and derating, move to the Alternator Output Margin without changing this page's inputs.
Before using this result
What measurement source fits Bank 1 short-term trim when it represents current short-term correction for bank 1?
Because bank 1 short-term trim represents current short-term correction for bank 1, use a source tied to the exact vehicle, component, and operating period described by the other fields.
How does the warning “Interpretation depends on operating mode, load, temperature, fuel system, and manufacturer strategy” affect Average combined fuel trim?
The condition “Interpretation depends on operating mode, load, temperature, fuel system, and manufacturer strategy” is not corrected automatically by the numeric inputs, so create a separate obd fuel trim case when it changes.
What assumption is expressed by “combined bank trim = short-term trim + long-term trim”?
In “combined bank trim = short-term trim + long-term trim,” bank 1 short-term trim and bank 1 long-term trim are treated as parts of one vehicle case.
How narrowly is Bank 1 long-term trim defined by “Learned long-term correction for bank 1”?
The definition “Learned long-term correction for bank 1” excludes a similarly named rating or a measurement taken at another reference point.