P0174 code — The most likely causes and fixes

Learn what a P0174 code means and the most likely causes

The definition of a P0174 code is System Too Lean (Bank 2). Bank 2 is the bank of cylinder that does NOT contain cylinder #1. In this case, the oxygen sensor upstream from the catalytic converter is sensing a lean condition.

Here’s a list of the most common causes of a lean condition

In simple terms, it’s caused by too little fuel or too much air. Here The following are some of the most common causes of this P0174 code:

Vacuum leaks: Vacuum leaks are a major cause of this code. Disconnected vacuum lines, a tear in the air duct from the air filter box to the throttle body, intake manifold gasket leak, or a leaking PCV hose are common causes of this code. If you’ve recently had an oil change, chances are the technician disconnected vacuum lines in order to check the condition of your engine air filter. They often forget to replace all the lines, causing a P0174 code.

Exhaust leaks are a common cause of P0174 trouble codes

Exhaust leak: An exhaust leak ahead of the catalytic converter is the second most common cause of this code. Any unmetered air that comes in contact with the oxygen sensor will cause it to read as if the exhaust is lean.  The most common exhaust component to cause this kind of leak is the flex pipe. See this post to learn more about the flex pipe and how it fails.

Faulty mass airflow sensor: The mass airflow sensor calculates the amount of air entering the engine and sends this information to the Engine Control Module (ECM). If the sensor is dirty or faulty, it can send incorrect information to the ECM, causing the P0174 code to appear.

Less common causes

Low fuel pressure due to a weak fuel pump: A weak fuel pump can cause the P0174 code to appear. If the pump is not supplying enough fuel to the engine, the air-fuel mixture can become too lean.

Clogged or dirty fuel injectors: Fuel injectors are responsible for delivering fuel to the engine. If they become clogged or dirty, they can cause the air-fuel mixture to become too lean, resulting in the P0174 code.

Clogged fuel filter: The fuel filter is responsible for filtering out impurities from the fuel before they reach the engine. If the fuel filter becomes clogged, it can restrict the flow of fuel to the engine, causing the air-fuel mixture to become too lean

Symptoms of a P0174 code

• Check engine light
• rough idle
• stalling engine
• misfire
• loss of power/lack of acceleration,
• increased fuel consumption

How to diagnose this code

• Start by checking for disconnected vacuum hoses and a disconnected or torn air duct from the air filter box to the throttle body.

• Check for debris buildup on the MAF sensor. Clean the sensing elements using MAF sensor cleaner. See this post on how to clean a MAF sensor.

• Check for exhaust leaks using a shop vacuum. See this post on how to find and exhaust leak

• Check the throttle body for carbon buildup and clean if necessary. Perform a throttle body relearn afterward.

• Connect a scan tool and perform a propane enrichment procedure to check the operation of the oxygen sensors.

© 2012 Rick Muscoplat