#fueltrim #p0171 #p0172 #systemlean #systemrich #oxygensensor

Most technicians understand the concept of fuel correction and what we mean by “closed loop” operation but just in case…

Fuel control is the concept of maintaining the proper mixture of fuel and air in the cylinder for efficient combustion. Incoming air is measured (directly or indirectly) by the Engine Control Module (ECM) and then the ECM chooses how long to open the injectors in order to add the proper amount of fuel to that air charge. The original injection time is based (in part) on the known flow of fuel through the injector but that original number is not perfect.

So a feedback sensor is used to tell the ECM how it did. These are the conventional oxygen sensor or the wideband oxygen sensor. Based on this feedback, the ECM adjusts the next injection event by increasing or decreasing the injector on time as appropriate. This process is continuous and results in the Short Term Fuel Trim (STFT) numbers you see on your scan tool.

Ideally, the ECM will adjust the on time to cause the STFT to swing from a positive number to a negative number with each adjustment, with an average approaching zero. If the adjustments the ECM has made are insufficient to do that, the ECM “learns” a more permanent correction called Long Term Fuel Trim and adds that factor to its internal calculations. If the total amount of correction (STFT + LTFT) exceeds a programmed threshold, the ECM will record a “system lean” or “system rich” DTC and turn on the Check Engine light.

But do you really understand the diagnostic advantages these Parameter Identifiers (PIDs) have to offer? To the knowing eye, the trims can point to the cause of that lean or rich system DTC. They can also be helpful in determining the cause of a misfire, or even identify a clogged exhaust.

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