Fault code P0174 – system too lean (Bank 2)

Fault code P0174 is called “System Too Lean (Bank 2)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0174

This Diagnostic Trouble Code (DTC) is a general powertrain code. Error P0174 is considered a general code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly depending on the model.

Fault code P0174 – system too lean (Bank 2)

For proper operation, the engine must add the correct amount of fuel to the amount of air intake. The correct fuel-to-air ratio is necessary for optimum power and fuel efficiency.

The engine control unit (ECU) uses a number of sensors to determine how much fuel to add. Most often, the air to fuel ratio in the mixture is determined based on oxygen sensor readings by calculating the amount of oxygen and carbon monoxide in the exhaust gases.

The most optimal ratio of air to fuel in the mixture fed into the engine cylinders is 14.7:1. This is the ratio needed to achieve maximum engine power and optimum fuel consumption.

In normal operation, the ECU monitors the balance between rich and lean mixture. At the same time controlling the short term and long term fuel balance (STFT and LTFT). Normally the fuel correction is 3% of the ideal value. It depends on the current air temperature, coolant temperature and load.

Engine problems such as high fuel pressure, a distorted MAF signal, or a faulty fuel injector can increase fuel consumption. Because of these problems, improper air flow data is coming in, which will result in distorted data.

The ECU will attempt to compensate for the imbalance in the fuel mixture. If the ECU fails to maintain the 14.7:1 ratio, it will trigger fault code P0174 – too lean fuel mixture (Bank 2).

Symptoms of vehicle malfunctions

The main signal that an error P0174 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. Check engine control lamp on the control panel will light up (the code will be recorded in the ECM memory as a malfunction).
  2. Floating revolutions, vehicle engine may be unstable.
  3. Decrease of the engine power.
  4. The engine may stall or fail to start.
  5. Increased fuel consumption.
  6. Possible ignition gaps in the engine cylinders.

The error P0174 is of a higher than average severity, therefore it is recommended to eliminate the problem as soon as possible. Most likely, with this problem you can continue driving, but the fuel consumption will increase. And because of poor combustion, there may be problems with the catalytic converter in the future.

Factors that can cause this error code

The error code P0174 can mean that one or more of the following problems have occurred:

  • The mass air flow sensor (MAF) may be dirty or damaged.
  • The air filter is dirty or contains oil.
  • Possible vacuum leak.
  • Fuel pressure problem.
  • Damaged fuel injector.
  • Worn spark plugs.
  • Fuel pump check valve may have failed.
  • PCV valve is leaking or has failed.
  • EGR valve may have jammed.
  • Defective oxygen sensor (Bank 2).
  • EVAP fuel vapor valve leaking.

How to fix or reset OBD-2 code P0174

Some suggested steps for troubleshooting and fix the error code P0174:

  1. Inspect all vacuum and PCV hoses and replace if necessary.
  2. Clean the mass air flow sensor. It is best to remove it and treat it with electronics cleaner or WD Be careful not to damage the mass air flow sensor, make sure it is dry before reinstalling.
  3. Inspect the fuel lines for cracks, leaks or pinching.
  4. Check the pressure in the fuel rail.
  5. Check the fuel injectors, they may be dirty. Use a fuel injector cleaner or have them professionally cleaned/replaced.
  6. Make sure there is no exhaust leak in front of the first oxygen sensor.

Diagnose and repair of problems

Diagnosing fault P0174 should be started by checking for contamination of the MAF sensor. Contamination can distort air intake measurements and therefore distort fuel injection calculations.

Next, check the fuel pressure, making sure it is in the correct range. Excessive pressure, due to a faulty pressure regulator or compressed line pipe, will cause more fuel to be injected than the ECU expected.

Check that the ECT and IAT (inlet air temperature) readings are correct. The ECU can pick up blocked sensors, so it is a good idea to check that the ECT and IAT readings are within the normal range.

For example, if the engine is running for half an hour on a hot day, the ECT should read about 176°F. If it is still showing 32°F, the ECU will enrich the fuel-air mixture to compensate, resulting in a rich condition.

Fuel pressure reduction

With the engine running, record the fuel pressure and then turn off the engine. The fuel pressure may drop slightly, but should remain stable for at least 10 or 15 minutes.

If the fuel pressure continues to drop, you may have a leaking fuel injector, resulting in a rich and possibly unflammable condition.

Blocked valve

A faulty valve in the block head, discharges unburned fuel into the exhaust stream. Check and correct this problem before attempting to correct a rich fuel-air mixture error.

Pay attention to other fault codes that may be present at the same time as error P0174. For example, for VVT (variable valve timing) or ignition system.

Vehicle exhaust

Check the exhaust system for leaks, especially upstream of the first O₂ sensor. Damage can be mechanical, in the form of cracks or caused by corrosion.

Atmospheric air in front of this sensor can distort the oxygen measurement, which can also cause code P0174. The ECU, in this case, will try to compensate the mixture and add fuel.

On which vehicles does this problem occur most frequently

Fault code P0174 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Acura
  • Audi (A6)
  • BMW (E39, E53, M54, X5)
  • Cadillac (SRX)
  • Chevrolet (Camaro, Epica, Silverado, Tahoe)
  • Chrysler (300C)
  • Dodge (RAM)
  • Ford (Expedition, Explorer, F-150, Focus, Maverick, Mondeo, Taurus)
  • Honda (Pilot)
  • Hyundai (Grandeur, Santa Fe)
  • Infiniti (QX56)
  • Isuzu (Rodeo)
  • Jaguar
  • Jeep (Grand Cherokee)
  • Kia (Sorento)
  • Land Rover (Range Rover)
  • Lexus (LX470, RX300, RX330, RX350)
  • Lincoln (Town Car)
  • Mazda (MPV, Tribute)
  • Mercedes-Benz
  • Mitsubishi (Montero, Outlander, Pajero)
  • Nissan (Maxima, Murano, Primera, Skyline, Teana, X-Trail)
  • Opel
  • Peugeot
  • Rover
  • Subaru (Tribeca)
  • Suzuki (XL-7)
  • Toyota (Avensis, Camry, Highlander, Ipsum, Land Cruiser, Mark II, Prado, Sienna, Tundra)
  • Volkswagen (Touareg)
  • Volvo
  • VAZ

Fault code P0174 can sometimes be found with other errors. The most common are the following: P0102, P0151, P0154, P0171, P0183, P0194, P0300, P0327, P0351, P0352, P0353, P0354, P2106, P2195.


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