P0171: System Too Lean Error Code. Causes and Fixes for P0171

Error code P0171 occurs when the mixture is too lean, meaning there is insufficient fuel and an excess of air. This mixture appears when the engine takes in a large amount of ambient air and/or the fuel system does not inject enough fuel.

This mixture condition leads to misfires, delayed throttle response, jerking during acceleration, dips in idle speed, and unstable engine operation. In Euro 2 and higher engine management systems, the Lambda sensor controls the mixture quality, which should have 14.7 parts air to one part fuel. When the air content exceeds the permissible range by 20-30% from the norm, error P0171 appears.

How the car behaves with a lean mixture

When running on a lean mixture, the engine runs unevenly at idle, overheats due to the slower combustion speed of such a mixture, and popping sounds (backfiring) can be heard in the intake manifold. The car itself may feel sluggish and gain revs slowly under load.

Prolonged driving with a lean mixture results in increased fuel consumption due to slow acceleration, forcing the driver to use lower gears. A lean mixture is caused by an imbalance in air intake and fuel delivery, resulting in too much air.

Why error P0171 appears

The causes of excessive air intake can be:

• Dirt on the Mass Air Flow (MAF) sensor, preventing it from correctly calculating air flow;
• Vacuum leak;
• The EGR valve, responsible for exhaust gas recirculation, not closing tightly and leaking air into the intake manifold;
• High air flow through the EGR valve due to a faulty EGR differential pressure sensor.

If the cause of the lean mixture is a lack of fuel, the faults are:

• Weak fuel pump;
• Clogged fuel filter with high resistance, restricting fuel flow;
• Dirty fuel injectors;
• Leak in the fuel pressure regulator.

To find the cause of error P0171, you need to outline the reasons why a lean mixture is entering the engine and triggering the Check Engine light. A sequential inspection plan is then drawn up:

1. Injectors and fuel delivery system;
2. Airtightness of the intake and exhaust manifolds and associated hoses;
3. Air flow and oxygen sensors (MAF and Lambda sensor).

Checking injectors, MAF and Lambda sensor

How to perform the check

Check the sensors. One of the most common causes of error P0171 is a contaminated Mass Air Flow (MAF) sensor. The sensor's reaction speed to airflow changes may be slowed down by dirt accumulated on its sensing element. It can also be contaminated by fuel vapours escaping through the intake manifold and throttle body when the engine is off. These vapours deposit a waxy layer on the sensor's element, leading to a signal indicating a lack of air. The ECU (Engine Control Unit) then does not add fuel when air increases, disrupting the overall mixture balance.

All this leads to a lean fuel mixture and the appearance of error P0171. Often, along with this error, codes P0100 or P0102 may appear, clearly indicating a MAF sensor malfunction. To solve the problem, the sensor must either be cleaned using a special electronics cleaner or carburettor cleaner (spraying onto the sensing element), or replaced with a new one.

Check for vacuum leaks. If the MAF is not the cause, check for vacuum leaks. De-pressurisation can occur at any point in the intake tract, as well as at the throttle body outlet – at the connection point of vacuum hoses and the intake manifold, the throttle body gasket, or intake manifold gaskets.

You should also inspect the PCV (Positive Crankcase Ventilation) hoses, fuel vapour recovery system hoses and intake manifold plugs for physical damage. The entire exhaust system near the oxygen sensor (half a metre before and after it) must be airtight. Otherwise, sensor readings may be incorrect (for example, the error can occur when the flex pipe burns through). Leaks are easiest to check using an OBD scanner (by checking fuel trim values).

If the car is fitted with an EGR differential pressure feedback sensor (DPFE), code P0171 can also appear if it malfunctions.

This sensor is mounted on the engine and connected by two hoses to the pipe that carries exhaust gases to the EGR valve. Corrosion in the sensor reduces its sensitivity, causing it to send information about a lack of gases. Consequently, the EGR valve remains open longer, leading to increased air levels in the mixture and a lean condition.

Check the fuel system. As mentioned, the P0171 lean mixture error code can also appear if there is insufficient fuel volume saturating the mixture. To check the fuel system health, the following tests are performed:

1. Check injector performance (on a test bench);
2. Check fuel pressure;
3. Check fuel pump performance;
4. Check voltage supplied to the fuel pump;
5. Check the fuel pressure regulator;
6. Check the condition of the fuel filter.

These tests can also be performed using a car scanner. The fuel pressure parameter and fuel trim values are checked.

If error P0171 occurs intermittently rather than constantly, causes may include unreliable contact in the sensor wiring harness and controller, mechanical damage to the harness, or poor grounding of the controller.