It’s the electronic control unit, and it monitors nearly every part of a car’s operations, from the air-fuel mixture to how much are is in the tires. The ECU should be able to open the fuel injectors more to allow for higher fuel levels in the mixture. Running lean means that the engine doesn’t have enough fuel to match the levels of oxygen that it’s receiving. This can happen for a variety of reasons, but in a normally functioning system, the ECU will tell it to cut back on the fuel flow to compensate. Running rich means that there is too much gas in the air-fuel mixture. The car’s electronic control unit (ECU) is the brain that controls the mixture and makes changes to the delivery of either component, based on its assessments. This ratio is referred to as the air-fuel mixture and is monitored by various sensors. Your car’s engine requires a specific balance of air and gasoline to run properly. If the system is expecting a certain level of fuel or air and does not receive it, the air-fuel mixture could be disrupted. Sensor failure could lead to the wrong fuel mixtures from the ECU. The mass airflow sensor monitors and communicates the levels of oxygen entering the engine. If an oxygen sensor fails, the ECU might not be able to make the correct decisions on the air-fuel mixture. If there’s a malfunction in the fuel system, the levels of fuel entering the engine can be too high or too low, which would cause a rich or lean scenario. In general, if you’re seeing issues with the air-fuel mixture, it’s caused by one of these things. Unraveling the causes behind lean or rich fuel mixtures can be frustrating, but it’s not impossible. If the value is negative, it indicates that the ECU is decreasing the flow of fuel into the injectors because the data it sees shows that the air-fuel mixture is too rich. If the value is a positive number, it means that the ECU is adding more fuel to the mixture to enrich it because the information it’s receiving indicates that the air-fuel mixture is too lean. A scanning tool can show the short- and long-term fuel trim numbers as a percentage. Your vehicle is constantly monitoring its air-fuel mixture and is making adjustments to keep it as close to the ideal range as possible, so you absolutely need all that data. Long-term fuel trim refers to the measurements these sensors make, which are effectively an average of the modifications the ECU made as a result of short-term fuel trim. While the sensors’ signal will fluctuate frequently upstream, where short-term fuel trim is measured, the signal at the sensors further down should be fairly constant. Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter. This immediate response is why it’s called short-term. In a properly functioning system, the ECU will react and immediately make changes as the exhaust gases change. An oxygen sensor monitors the flow and generates a signal that is proportional to the oxygen levels in the gases. Short-term fuel trim happens as a result of changes in the oxygen levels of exhaust gases. In short, fuel trim describes the adjustments that a vehicle’s engine control unit (ECU) makes to the fuel delivery system to keep the air-fuel mixture as close to its ideal ratios as possible. The Drive’s fluid dynamicists have done the calculations to bring you the basics of fuel trim, why and how it happens, and what you can do to fix it if there’s a problem. But it’s not too late to learn the ins and outs of fuel trim! We know you’re champing at the bit to do just that. It’s totally fine if you’ve never heard of this concept before-many haven’t. This process, known as both short- and long-term fuel trim, is how the ECU keeps track of how much fuel is flowing in. Quarterbacking the whole game, the ECU monitors the flow of both air and fuel into the engine and directs changes to either part to ensure the proper mixture. The fun part of all of this is that there are several sensors and your car’s electronic control unit (ECU) that are in charge of monitoring the whole process, and the failure of any part of the system can cause issues. Too much air, too little fuel, a field mouse making the engine bay it home, and things can start to get dicey. Your car’s engine is an amazing feat of engineering, but it doesn’t take much to cause problems with the way it runs. The Drive and its partners may earn a commission if you purchase a product through one of our links.