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P0171 and P0174 – lean mixture codes in Ford cars

P0171 and P0174 - lean mixture codes in Ford cars 1

P0171 and P0174 – lean mixture codes in Ford cars

P0171 and P0174 – lean mixture codes in Ford cars

On Ford vehicles, error code P0171 means too lean mixture (too much air, not enough fuel) in the 1st cylinder block, code P0174 means too lean mixture in the 2nd cylinder block.

When the “Check Engine” indicator lights up, either of these codes, or one of them can be detected during car diagnostics by a scanner or a code reader connected to the car’s diagnostic connector. If the mileage is large enough, both codes are likely to appear..

P0171 – lean mixture code for the 1st cylinder block located RIGHT (passenger side) in Ford vehicles with a V6 or V8 engine and rear-wheel drive.

P0174 – lean mixture code for the 2nd cylinder block located LEFT (driver’s side) in Ford vehicles with a transverse engine and front-wheel drive. In cars with four-cylinder engines (without a second cylinder block), this code does not occur.


This state of the mixture indicates that the engine draws in too much air and / or the fuel system does not supply enough fuel. Too lean air-fuel mixture can cause misfire, uneven idle, delays or twitching during acceleration and / or poor engine performance.

Excess air can enter the engine due to the following reasons: vacuum leak; a dirty air flow sensor that is not able to correctly record the air flow; the exhaust gas recirculation (EGR) valve does not close and passes exhaust gases into the intake manifold; too high flow through the EGR valve (EGR differential pressure sensor is faulty and underestimates the consumption of recirculated exhaust gases).

If the problem is a lack of fuel, then its source may be a weak fuel pump, a fuel filter with resistance, a leaking fuel pressure regulator, or dirty fuel injectors.

Clogged Mass Air Flow (MAF) Sensor

One of the most common causes of the P0171 and P0174 codes in Ford vehicles is the clogged mass air flow (MAF) sensor. This sensor is located in the air inlet pipe in front of the throttle body. The MAF sensor must be protected from external dust and dirt with an air filter, but sometimes the air filter does not fit snugly enough and passes unfiltered air into the engine.

A dirt layer may form on the sensor wire, slowing down its response to changes in air flow. The mass air flow sensor may also be contaminated with fuel vapors that return through the intake manifold and throttle body when the engine is off. Fuel vapors leave a layer of paraffin on the sensor wire, due to which the sensor sends an underestimated air flow rate to the control unit, and the control unit, in turn, does not add enough fuel to maintain an optimally balanced mixture. As a result, the air-fuel mixture is leaner and code P0171 and / or code P0174 is generated (for more information see TSB Technical Service Bulletin 98-23-10 for Ford vehicles).

It is possible to diagnose pollution of the mass air flow sensor using a scanner: go to the parameter identification data (PID) menu and check the fuel balancing values ​​when the engine is running. If the sensor is dirty, balancing the fuel production will be close to normal, (plus or minus 3-5), but with an increase in the number of revolutions to 2500 rpm, the fuel injection value will be positive (5 or higher).

This problem can be solved quite simply: a contaminated mass air flow sensor should either be cleaned or replaced. In many cases, the sensor is successfully cleaned with a cleaning tool for electrical appliances that is sprayed onto the sensor. Use only this type of cleaning agent – any other can damage the sensor..

Disconnect the air inlet pipe, which is directly in front of the sensor, and spray the cleaning agent through the screen and the wiring element into the center of the small MAF sensor. Allow the product to blend in for a few minutes and then spray again. Wait five minutes and you can connect the air inlet pipe and start the engine.

If the lean mixture codes reappear, but the engine does not have a vacuum leak or problems with the fuel supply, you may need to replace the mass air flow sensor.


Another common reason for the appearance of codes P0171 and P0174 in Ford cars is vacuum leakage. Vacuum leaks can occur anywhere in the inlet pipe at the outlet of the throttle body (in the gasket of the throttle body, in the gaskets of the inlet manifold, at the junction of the vacuum hoses with the intake manifold).

You can use a scanner to diagnose a vacuum leak. Connect your autoscanner, start the engine, go to the parameter identification data (PID) menu and check the idle fuel balance values. If a vacuum leak is present, the balancing value of the fuel production will be positive (5 or higher). Now accelerate the engine to 2500 rpm. If the problem is in the vacuum leak, then it will make itself felt less at high speeds and under heavy loads, and the value of balancing the fuel production will approach normal (closer to zero, plus or minus 3-4).

Refer to TSB 04-17-4 for Ford Automotive Technical Service Bulletin for fuel balancing and vacuum leak detection procedures..

In Ford cars with a 3.8-liter engine with a split-chamber intake manifold, the gaskets and insulation bolts for the upper manifold chamber wear out and allow air to pass over time, often because oil is sucked into the intake manifold through a forced ventilation system crankcase (PCV). In addition, the vacuum hose connecting the fuel pressure regulator to the intake manifold can expand and let the vacuum pass at the point where the hose connects to the manifold. According to TSB 03-16-1 Technical Service Bulletin for Ford vehicles, solving the problem involves several steps: remove the upper manifold chamber and replace the original gaskets and bolts with modified ones; replace the front valve cover with a modified cover that reduces the amount of oil vapor absorbed into the crankcase ventilation system; inspect and replace the fuel pressure regulator hose; and finally, reflash the control unit so that it is less sensitive to the lean state of the fuel-air mixture.


Poor mixture codes can also occur if the engine receives insufficient fuel. The reason for this may be a weak fuel pump, too low a voltage supplied to the fuel pump (because of which it cannot rotate fast enough to ensure a normal fuel supply), a fuel filter with a resistance or, possibly, a leaking fuel pressure regulator. For more information on identifying problems in the fuel supply system, see the article “Diagnosing the malfunction of the fuel pump”.

You can use a scanner to diagnose fuel problems that result in poor mixture codes. Connect the scanner, go to the parameter identification data (PID) menu and check the identifier of the fuel pressure parameter. If the fuel pressure is lower than the value specified in the specifications, then most likely the problem is in the fuel pump or in the wiring circuit of the fuel pump.

Then check the fuel trim values ​​with the engine running. At idle, balancing fuel production will be close to normal. If the engine has problems with fuel supply, the fuel balancing values ​​will increase as speed and load increase. The absence of changes in the fuel balancing values ​​will tell you that the engine is getting enough fuel and that the reason for the poor mixture code is NOT in low fuel consumption.

Contaminated fuel injectors can affect the fuel supply system as much as a weak fuel pump. At idle and at low speeds, they inject enough fuel to meet the needs of the engine, but at higher speeds and loads they can not cope with their task. The idle fuel balancing value will be close to normal, but as speed and load increase, this value will begin to increase..

Cleaning the fuel injectors will solve this problem. If the nozzles are very dirty, the fuel additives may act too slowly or inefficiently, so it may be necessary to have the nozzles cleaned for professional cleaning..


The cause of the codes P0171 and P0174 in Ford cars may also be a malfunction of the differential pressure sensor in the exhaust gas recirculation system. The failure rate of such sensors is very high if the vehicle’s mileage exceeds 60,000 miles or if it has been in use for more than five to six years.

The differential pressure sensor is mounted on the engine and is connected by two rubber hoses to a pipe through which the exhaust gas is supplied to the valve of the recirculation system (EGR). The original sensor has a rectangular aluminum casing approximately 7.5 cm long. Corrosion inside the sensor reduces its sensitivity to the consumption of recirculated gases, as a result of which its readings are underestimated. The control unit responds to this by increasing the flow of recirculated gases, due to which the EGR valve remains open longer than usual, which leads to depletion of the fuel-air mixture in the engine. Thus, a faulty sensor can cause the code P0401 (insufficient flow in the exhaust gas recirculation system) or codes P0171 and P0174 (lean mixture).

In most cases, the code P0401 is generated due to a malfunction in the differential pressure sensor, and not due to a malfunction or clogging of the EGR valve (although this can also cause the appearance of code P0401). In the spare parts market, the differential pressure sensor costs less than $ 50, and if you install it instead of the old sensor, you will get rid of not only the P0401 code, but also the P0171 and P0174 codes.


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