P0174: System Too Lean on Bank 2

What is P0174?

Error code P0174 is an OBD-II diagnostic trouble code (DTC) defined as “System Too Lean (Bank 2).” It’s indicated via a lit check engine light (CEL) after your Engine Control Module (ECM) detects a lean fuel condition on Bank 2. 

How does that happen?

A system too lean condition occurs if there’s too much air and insufficient fuel in your air-fuel mixture. Usually, this means that your fuel system isn’t delivering enough fuel, and the fuel pressure is too low for the combustion process. 

Your ECM is designed to adjust the air-fuel ratio — but only in small increments. 

If your air-fuel mixture requires a higher compensation than usual, the ECM will trigger the CEL and set engine code P0174. An example where this can happen is when a vacuum line leak introduces unmetered air into the air-fuel mixture.

Common Symptoms

Many symptoms associated with the P0174 code are more noticeable at lower speeds and revolutions per minute (RPM).

If you have an active P0174 engine code, you may experience:

• Increased fuel consumption
• An illuminated check engine light
• Rough idle
• Engine misfires
• Engine coughing
• Decreased acceleration
• Engine stalling
• Low fuel pressure

Note: The P0174 engine code is very similar to code P0171. The P0171 code also indicates an issue with your air-fuel mixture but refers to Bank 1.

Can I Still Drive?

Yes, you can still drive.

Generally, a lean code isn’t that serious. However, air leaks in the intake manifold can let particles enter the engine and cause internal damage. Furthermore, driving with a prolonged lean fuel condition can lead to friction between some of the moving parts in your engine and damage critical components.

It’s best to head to a professional mechanic to get this code fixed ASAP.

P0174 Causes

Below are some common causes of the error code P0174:

• Vacuum leak 
• Malfunctioning mass airflow sensor (MAF sensor)
• Clogged fuel filter or weak fuel pump
• Dirty fuel injector
• Failing fuel pressure regulator 
• Exhaust leak
• Cracked throttle body gasket
• Faulty O2 sensor 
• Worn-out spark plug
• Engine Control Module (ECM) or Powertrain Control Module (PCM) issues

Diagnosis

Although you can diagnose the engine code P0174 yourself, you’ll need specialized equipment to read fuel trim values. Fuel trim values are what the ECM uses to compensate for all problems relating to air-fuel mixture ratios.

So, it’s best to leave the diagnosis to a professional.

Here’s what a mechanic would do:

1. Analyze the short–term and long–term fuel trim values, plus the freeze frame data. The short–term fuel trim values measure immediate changes to your car’s oxygen levels, while long–term values measure O2 levels over a longer period.

Note: Short–term and long–term fuel trim values help identify fuel system problems, like a weak fuel pump, exhaust leak, dirty fuel injectors, low fuel pressure, throttle body gasket leaks, etc. 

2. If you have a dirty MAF sensor, the fuel trim values will increase as your engine speed increases. The mechanic will test your mass air flow sensor using a scan tool.

3. Inspect the air filter to ensure it’s not letting dust or debris clog up your MAF sensor.

4. Check for fuel system issues, such as a weak fuel pump, by performing a fuel pressure regulator test.

5. Inspect your fuel injector and fuel filter components to see if they’re dirty or clogged.

6. Check if a leak in your intake system or vacuum line allows excess air to enter your engine. Leaks can occur due to a damaged intake manifold gasket or punctured vacuum hose.

7. Inspect your spark plug wells and rubber grommets for oil. Defective valve covers can also cause an oil leak. Any oil contamination can lead to the engine running leaner than intended. 

8. Check for a defective PCV valve or EGR valve —  which can throw off your air-fuel mixture.

9. Use a scan tool to test the air-fuel ratio relayed by the oxygen sensor.

Your mechanic must complete the diagnostic process before attempting repairs. You don’t want to pay for a new MAF sensor when your faulty O2 sensor is the root cause of the lean fuel condition.

Possible Repairs for P0174 & Costs

Since several issues can cause lean code P0174, there isn’t just one possible solution. 

Here are some fixes your mechanic may perform:

1. Check for other error codes: Your mechanic will use a scan tool to ensure no other error codes indicate problems (that may or may not be related to the P0174 code.)

2. MAF sensor issues: If it’s a mass air flow sensor problem, they’ll clean your mass airflow sensor with a MAF cleaner or replace the sensor if it’s broken.

3. Clogged air filter: A dirty air filter could contaminate your MAF sensor, so they’ll clear it out if needed.

4. Faulty oxygen sensor: Your mechanic will replace your O2 sensor if it’s defective.

5. Fuel system issues: They’ll inspect your fuel system and fix problems such as a dirty fuel injector, weak fuel pump, or clogged fuel filter.

6. Vacuum leaks: If there’s a vacuum leak, your mechanic will inspect your vacuum hose and intake manifold gasket and replace them if needed.

7. Faulty EGR valve or PCV valve: If there’s still too much air causing a lean fuel mixture, they’ll replace your EGR valve and PCV valve.

8. Defective valve cover gasket: If there’s an oil leak due to a damaged valve cover gasket, they’ll replace it with a new one. 

Your mechanic will need about an hour to diagnose the issue before they give you a repair estimate.

Here are cost estimates depending on the issue (including labor costs):

• PCV valve replacement: $75 to $100
• MAF sensor cleaning: $100
• Fuel filter replacement: $100 to $200
• Spark plug replacement: $150
• Air fuel sensor or oxygen sensor repair: $200 to $300
• Fuel pressure regulator repair: $200 to $400
• MAF sensor replacement: $300
• Exhaust leak repair: $300 to $400 
• Vacuum leak repair: $1000
• Fuel pump repair: $1,300 to $1,700