LTF and STF Fuel Trim Engine Codes P0170, P0175, P0172, P0174

LTF and STF Fuel Trim Engine Codes P0170, P0175, P0172, P0174

Understanding what parts should be replaced to get ride of these codes.

Codes P0170 through P0175 indicate issues with the fuel trim – six codes covering banks 1 and 2. Codes 170 and 173 indicate an issue with the fuel trim for which the ECM can no longer compensate. Codes 171 and 174 indicate a lean condition for banks 1 or 2. Codes 172 and 175 indicate a rich condition in either bank 1 or bank 2. 

What parts should you replace to get rid of these codes? Is it the oxygen sensors? No, because while it is possible that contaminated oxygen sensors can produce lower readings, the fact that the ECM was able to set this code indicates the oxygen sensors are functioning. 

Is it the fuel injectors? Maybe, but 99 percent of the time it is going to be a strong “no” if there are no codes precent for the injectors or fuel pump.

Codes P0170 through P0175 are caused by either unmetered air entering the engine or excess fuel entering the combustion chamber, though it could also be a case where air or fuel is unable to enter the combustion chamber.

The modern engine measures two things extremely well: the amount of air going into the combustion chamber with the mass airflow sensor (MAF) and the byproducts that are generated by the combustion event with the oxygen sensor. These measurements allow the engine’s computer to put the right amount of fuel and spark into the cylinder to give the most efficient and cleanest combustion event.

Fuel-injected engines can compensate for altitude, barometric pressure and engine loads by adjusting fuel trims. When there is less oxygen, the engine will need less fuel. Less fuel means a leaner fuel trim and less power. This adjustment comes at the detriment of performance by taking away fuel.

It also works in the opposite direction. If unmetered air makes its way past the MAF sensor, the oxygen sensor will detect a lean combustion event. The ECM will then instruct the injectors to pulse for a longer period of time. This increases fuel consumption and decreases fuel economy.

The ECM can compensate only so long before it becomes unbalanced and can no longer add or restrict fuel to achieve a proper oxygen sensor reading. This is when codes are set for a “too rich” or “too lean” fuel trim. 

You must understand that the ECM is either taking away fuel or adding fuel so the exhaust byproducts, measured by the oxygen sensor, add up to an efficient operation. A positive number means the fuel system is adding fuel by lengthening the injector pulse so more fuel goes into the combustion chamber. A negative number means the engine is taking fuel away by shortening the injector pulse.


The green line is the long-term fuel trim that does not change while the short-term fuel trims change. 
As the oxygen sensor reads a lean condition, the short-term fuel trim compensates, increasing the Positive Fuel Trim. 
Long-Term Fuel Trims can change with altitude.
If you increase the time base, you can see
changes to the Long-term fuel trim. 

Accurate fuel trim values require an accurate feedback signal from the oxygen sensor, otherwise the engine computer has no way of knowing whether the fuel mixture is running rich or lean. When the engine is shut off, the fuel trim values are retained in the computer’s memory so the next time the vehicle is driven it can pick up where it left off. 

Erasing the computer’s memory with a scan tool, or disconnecting the battery or the PCM power supply to clear codes, also wipes the fuel trim values, which means the computer has to start learning the fuel adjustments all over again the next time the engine runs.

The engine must be started and running to read the fuel trim information. Choose the option that allows you to read system live data, which varies depending on the scan tool. This will display a long list of sensor outputs and other readings called PIDs (Parameter IDs). There will be two fuel trim values:

Short-Term Fuel Trim (STFT) values change rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions. Readings may jump as much as 25% or more in either direction depending on loads, throttle position and engine speed.

Long-Term Fuel Trim (LTFT) is a longer-term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time. Values normally range from -10% to +10%. This value is a more accurate indicator of how the fuel mixture is being corrected to compensate for changes in the air/fuel ratio that are occurring inside the engine.

With either fuel trim, it is critical to see that the system is making corrections to bring the engine into the correct fuel trim for efficient operation. The ECM is not immediately reacting to fuel trim; instead it is reacting and correcting and then measuring it.

If you see a fuel trim pegged high or low at the maximum limits, there could be problems like an intake manifold leak or a fuel injector that is stuck open. Fuel trims are not the final word for diagnostics. As for codes P0170 through P0175, they are only a starting point.

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