OBD2 error P0174 possible causes

The reasons that may lead to the establishment of code P0174 may be the following:

• Intake air leaks
• Defective first heated oxygen sensor (lambda sensor)
• Abnormal combustion
• Defective fuel injectors
• Exhaust gas leaks
• Abnormal fuel pressure
• No fuel
• Defective air mass flow sensor (MAF)
• Incorrect connection of the crankcase venting hose (PVC)

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Technical notes

If P0171 is combined with code P0174, it is very likely that the problem is caused by an intake air leak. If there is no inlet leakage, the next step is to replace the air filter and clean the flow meter. If the problem persists, you may need to replace the front oxygen sensor (O2).

When will the code be detected?

• The fuel injection system is not working properly (Fuel mixture ratio is too low).

Possible symptoms

• Fault light on
• Excessive fuel consumption

What does code P0174 mean?

Long-term fuel equalization and short-term fuel equalization provide the necessary diagnostic data to diagnose engine problems. In fact, reading the long-term fuel mix on a diagnostic tester can lead to a quick determination of code P0174 or P0171.

Fuel trimmers are settings that control the amount of fuel entering the engine to achieve a stoichiometric ratio of 14.7: 1. The stoichiometric ratio is a volumetric mixture of air and fuel that will result in complete combustion of fuel and oxygen. The car’s steering computer will always look for 14.7 litres of air for every litre of petrol. It is important to remember this basic right for your car.

Since stoichiometric ratio is very important for engine performance, there are many air sensors and fuel controls that measure the air and fuel entering the car engine. The most common sensors are the mass flow sensor (MAF), the intake air temperature sensor (IAT), the intake manifold air pressure sensor (MAP), short-term fuel consumption correction (STFT), long-term fuel consumption correction (LTFT) and the oxygen sensor (02). A few examples will explain the interaction of these sensors and their role in engine diagnostics.

What are the most common reasons for code P0174?
Unmeasured air entering the engine is a common cause of code P0174. While the air passing through the MAF sensor is measured by the car computer, intake or vacuum leaks are unknown. These additional air sources increase the air-fuel mixture. A quick way to detect a leak is to observe LTFT readings at idle and full throttle. When the throttle is fully open, a small leakage will reduce the total amount of air entering the engine much less.

The poor condition, which can lead to error code P0174, is much more common than the rich condition. The poor condition is detected when the O2 sensor reacts to the presence of oxygen, inhibiting its output voltage. In other words, the oxygen sensor will produce 1 volt if the exhaust mixture does not contain oxygen. A typical 02 sensor reading for a poor condition will be much lower than 1 volt, as shown below.

When the car computer detects a poor condition in the O2 sensor, it will immediately try to correct the poor condition and return to stoichiometric ratio by adding fuel. Enter the fuel mixture short term setting (STFT). The short term fuel equalization is the percentage change of fuel delivered to the engine. If the mixture is poor, more fuel is needed and the short-term fuel correction will be positive between 0 and 50%. The short-term fuel equalisation increases until the oxygen sensor informs the computer that there is a stoichiometric ratio of 14.7. When the engine returns to stoichiometric equilibrium, the short-term fuel equalisation increases. Now enter the long-term fuel mixture setting (LTFT). LTFT will be set to STFT. LTFT will take over STFT (which will return to zero after LTFT stores the STFT setting) to keep the engine in stoichiometric ratio.

What causes code P0174? In general, if LTFT is greater than 25% for two driving cycles, the computer will discard the code P0174.

P0174 Description

By using air/fuel self-checking control, the actual mixture ratio can be similar to the theoretical mixture ratio based on the mixture ratio feedback signal with heated oxygen sensors 1. The engine control module (ECM) calculates the necessary correction to correct the shift between the actual and theoretical ratio. In case the compensation value is extremely high (the actual mixture ratio is too low), the ECM evaluates the condition in which the fuel injection system is malfunctioning and illuminates the MIL control