The overall engine operation relies on the information from the engine control module (ECM). One of the main functions of the ECM is to control the air-fuel mixture ratio — which can be short-term fuel trim or long-term fuel trim. What is the short-term fuel trim normal range, anyway?
The STFT B1 normal range of the engine control module is between -22% to +25%. Any value above or below this means there’s something wrong with the engine or the sensors are bad and it should be checked and addressed.
A good way to fully understand this topic is by understanding what fuel trim is and how it works.
What is fuel trim, and how does it work?
For any system in a car to function correctly, all components must be in good working condition. The fuel system is no exception — there must be a balanced air-fuel ratio for effective engine operation.
The car computer or the engine control module relies on the data from the MAF and O2 sensors to determine the right amount of fuel and air to send into the engine for optimum combustion.
If the ECM determines there’s a lean or rich condition based on the information from these systems, it’ll make appropriate adjustments. If it finds there’s too much air in the engine, it’ll add extra fuel to balance the air-fuel ratio. Also, if it finds too little air, it’ll reduce the fuel input to balance the ratio.
Once the O2 sensor detects too much air and too little fuel in the exhaust system, it’ll report to the car computer in a positive percentage value. To adjust this, the ECM will increase how long the fuel injectors stay open until the O2 sensor reports a normal reading.
On the other hand, if there’s too much fuel and too little air in the exhaust, the O2 sensor will report to the car computer in a negative percentage value. The computer will decrease how long the fuel injectors stay open until the O2 sensor reports a normal reading.
The process of increasing or decreasing the amount of fuel that enters the engine is known as fuel trimming. The data calculated by the ECM will be compared with the amount of fuel that enters the engine using a predefined air-to-fuel ratio by the manufacturer. The calculated data must not be too little or too much.
But here’s the thing: fuel trimming comes in short-term and long-term trims. Long-term fuel trim normal range will be a topic for another day. So, let’s focus on short-term fuel trims.
What is short-term fuel trim (STFT)?
Short-term fuel trim is the short-lived and immediate response of the ECM to the changes in the exhaust gasses. This means that the STFT depends on the ECM response as it receives data from the O2 sensor.
The engine control module STFT ensures the engine maintains a balanced air-fuel ratio. It achieves this by constantly adjusting the duration of the fuel injection during different engine operating conditions, such as transient and warm-up periods.
As the engine is warming up, it operates in open-loop mode — meaning it depends on sensor readings and predefined fuel maps instead of relying on the oxygen sensor alone. As the car engine gets to its average operating temperature, it changes to closed-loop mode — making the STFT more active. At this point, the engine control module collects real-time data from the O2 sensor to adjust the air-fuel ratio, ensuring optimal combustion and less emissions.
The STFT values keep fluctuating as the engine temperature, engine load, the quantity of oxygen in the exhaust, and as other factors affecting the air-fuel ratio changes. As the changes happen, the ECM swiftly adjusts how long the fuel injectors stay open to balance the air-fuel ratio.
What does positive STFT mean?
As explained earlier, short-term fuel trim is expressed in percentages. A positive short-term fuel trim means that the engine control module (ECM) has detected that there’s too little fuel in the exhaust gas, and it is attempting to richen the fuel mixture.
Negative STFT is the opposite of positive short-term fuel trim. This means that when there’s a negative STFT, the car computer has detected too much fuel in the engine and it’s decreasing the amount of fuel that enters the engine to maintain a proper air-fuel mixture ratio. STFT, whether positive or negative, are adjustments made by the car computer to maintain a predefined air-fuel mixture ratio, which is approximately 14.7:1.
Is fuel trim rich or lean?
Fuel trims can be either rich or lean. If the powertrain control module (PCM) or car computer detects lean conditions and is attempting to add more fuel to rich out the fuel mixture, it can be said to be lean. Similarly, if it detects rich conditions (too much fuel and too little air) and is trying to decrease the fuel input, it can be said to be rich.
What causes high STFT levels?
The common causes of STFT B1 high are malfunctioning MAF sensors, faulty oxygen sensors, and faulty fuel injectors. These issues can also cause high long-term fuel trim issues. To address the issue, you have to diagnose and replace defective components.
Faulty fuel injectors
A dirty or malfunctioning fuel injector will not work well. It’ll not be able to deliver the right amount of fuel needed in the engine. As a result, the powertrain control module will compensate by adding more fuel whenever it detects injector issues.
Malfunctioning O2 sensor
The O2 sensor measures the amount of oxygen in the exhaust and tells the powertrain control module to adjust the air-fuel ratio whenever it detects too much or too little air. A malfunctioning O2 sensor can send inconsistent and incorrect signals, causing the PCM to add more fuel than necessary. Invariably resulting in high short-term fuel trims.
Bad MAF sensor
The mass airflow sensor measures the air that enters the engine and tells the powertrain control module to adjust the air-fuel ratio whenever it detects that too much air is entering the engine. A lousy MAF sensor can send inconsistent and incorrect signals, causing the car computer to add more fuel than necessary, resulting in high STFT.
How to Read Fuel Trim
The process of reading fuel trim values is quite simple. Here’s how to read it.
- Start the car
- Locate the vehicle OBD2 port and connect the plug to your scanner
- Select the option that allows you to read the system’s live data
- The scanner will pop different sensor outputs and other readings called Parameter IDs (PIDs).
What controls STFT and LTFT?
The LTFT B1 range refers to the percentage of fuel trim over a long period of time for cylinder bank 1. The normal LTFT B1 range should be 0 or numbers near 0 if all components work fine in a normal engine working condition. However, the value can fluctuate since vehicles don’t move at a constant speed.
For example, while LTFT B1 normal range at idle could be close to 0, it should return to zero once the vehicle regains steady speeds. More so, these fluctuations should not be too rampant; if the values fluctuate like a STFT, there is an issue. If the sensors work correctly, the LTFT B2 normal range should also be 0 or near 0.
Wondering what B1 and B2 mean? Here you go. B means bank and represents each cylinder in your engine. With B1 standing for bank 1, that is the number one cylinder bank, B2 stands for bank 2 cylinder.
To therefore explain LTFT B1 meaning, it is the long-term fuel trim value on cylinder bank 1. What about the STFT B1 meaning? This indicates the short-term fuel trim value on cylinder bank 1.
What is the Normal STFT B1 range?
I mentioned earlier that the normal STFT B1 range of the ECM is -22% to +25%. But in actual sense, the range of the STFT to monitor is -10% to +25%, equivalent to +5% and +10%. The minus sign shows that the ECM is decreasing the fuel that enters the engine, while the plus sign shows that the ECM is adding more fuel to the engine.
Keep in mind, engine misfires may change the STFT values to +15 to +25. Plus, other parameters like the engine load, speed, and throttle lift-off may affect the values of the fuel trims.
Once the values go beyond STFT b1 normal range on Ford or your respective car model, it is a sign that there’s an issue with the sensors or the engine. Once the car computer detects this issue, it’ll log it as a trouble code. The ECM will log error code P0172 if the engine is running rich and P0171 if the engine is running lean.
What is STFT B1 S1 normal range?
STFT B1 S1 is a short term fuel trim for bank 1, sensor 1, which is the upstream oxygen sensor on the catalytic converter. The normal B1S1 voltages are any consistent fluctuation between .1 to .8V.
What is STFT B1 S2 normal range?
STFT B1 S2 is a short term fuel trim for bank 1, sensor 2, which is the downstream oxygen sensor on the cat con. The normal range for short term fuel trims is 0, fluctuating from -25 to +25.
The default value for B1S2 is 99.2%. This value is for an O2 sensor that doesn’t contribute to the fuel control strategy. The scanner requests the value of the PID from the ECM, and the ECM is required to send something.
Short-term fuel trim is an essential parameter that ensures a balanced air-fuel mixture for optimal combustion. The STFT B1 represents the readings in engine cylinder bank 1. The only difference between STFT B2 normal range and STFT B1 is that B1 is talking about bank 1, while B2 talks about bank 2.
The STFT B1 normal range should fall between -10% to +25% under perfect engine operation. This range allows for swift and minor adjustments in air-fuel ratio variations. If the fuel trim falls outside this range, it shows there are issues with the engine, O2 sensor, or the MAF sensor.