What does it mean?
Your engine needs to put in the right amount of fuel for the amount of air coming into it, that is, the proper air-fuel ratio, to deliver the best power and fuel economy. The ECU (engine control unit) uses a number of sensors, such as the MAF, CKP, and ECT (mass air flow, crankshaft position, and engine coolant temperature) to determine how much fuel to put in.
Using feedback from the HO 2 S, AFR (heated oxygen sensor or air-fuel-ratio sensor), or Lambda sensor, the ECU can fine-tune fuel injector pulse-width (IPW) to deliver the best air-fuel ratio. As suggested by the names, the HO 2 S is sensitive to oxygen (O 2 ) content in the exhaust, which has a direct relationship to air-fuel ratio.
On combustion, some of the oxygen in the air is used to oxidize the fuel, leaving a small amount in the exhaust, which the HO 2 S measures, sending a signal back to the ECU. If there is a lot of oxygen, this means that the engine is running lean, that is, not enough fuel was injected. Conversely, if there is very little oxygen, this means that the engine is running rich, that too much fuel was injected.
Under normal operation, the ECU cycles between rich and lean air-fuel ratios, at the same time learning short-term fuel trim and long-term fuel trim (STFT and LTFT) values. Typically, fuel trims will be within 3% of the ideal, dependent on current air and coolant temperatures and load, for example.
On the other hand, engine problems, such as high fuel pressure, a skewed MAF signal, or a faulty fuel injector, may dump in more fuel than required, or misreport how much air is entering, resulting in a rich condition.
The ECU will try to compensate, by reducing IPW, but if fuel trim goes over -25% or so, the ECU interprets this as a malfunction, setting a DTC (diagnostic trouble code), DTC P0172 – Fuel System Too Rich (Bank 1), and illuminating the MIL or CEL (malfunction indicator lamp or check-engine light).