The on-board monitor responsible for keeping an eye on all this is the fuel system monitor, and it does its job continuously, watching for excessive corrections to the fuel quantity delivered by the Engine Control Module (ECM) and setting Diagnostic Trouble Codes (DTCs) when things get out of hand. Diagnosing fuel trim related faults using fuel trim Parameter Identifications (PIDs) can help us in diagnosing other driveability issues.

What is Fuel Trim?

The inside of a lambda sensor is a microcosm of technology at work.

When engineers are programming the ECM for a given engine, they are able to model the quantity of air flowing into that engine at various engine speeds (rpm) and throttle openings (load). These theoretical air charges are programmed into a map with corresponding fuel injection pulse widths to form an rpm/load look-up table that the ECM will use as a base to determine how much fuel to supply. To make sure the fuel charge is correct, the ECM relies on a feedback measurement of oxygen in the exhaust stream. If needed, it will then "'trim" the pulse width to maintain the correct value for lambda (the term used to define air/fuel ratio relative to stoichiometry).

A healthy lambda sensor is key in maintaining overall peak performance.

That seems like a fairly wide range, doesn't it? We still have to consider emissions, however, and to meet current standards, a three-way catalytic converter has to receive exhaust gases from a much narrower lambda range: 1.000 +/- 0.005! Lean of this range, oxides of nitrogen (NOx) emissions begin to increase. A mixture that's too rich can lead to increased hydrocarbon (HC) and carbon monoxide (CO) emissions, as well as potential catalyst damage.

On Board Diagnostic 2 (OBD2) regulations require that the ECM be capable of monitoring the systems that could cause variances in vehicle emissions, and report unacceptable variances to the driver by turning on the Malfunction Indicator Lamp (MIL). In addition to all of this, the designers have to take into account vehicle aging and its effect on lambda, building in a means to adjust the base fuel injector pulse width to compensate.

This adjustment is the definition of fuel trim. The ECM adjusts, or "trims" the injector pulse width to slightly increase or decrease the amount of fuel that would otherwise be injected based solely on the amount programmed.