P0173 Fuel Trim Malfunction (Bank 2): Causes, Symptoms, and Fixes

Quick answer

• What the code means: The P0173 code means that the engine's control module has detected a problem with the fuel trim on Bank 2, indicating the air-fuel mixture is too rich or too lean.
• Severity: It is safe to keep driving, but the issue should be addressed as soon as possible to prevent potential damage to the catalytic converter and to avoid poor engine performance and fuel economy.
• Most common cause: The most common cause of a P0173 code is a vacuum leak.

What the P0173 code means

The official OBD-II definition for P0173 is "Fuel Trim Malfunction (Bank 2)." In plain English, this means that your car's engine computer (ECU) is having trouble maintaining the proper air-fuel ratio on one side of the engine. "Bank 2" refers to the side of the engine that does not contain cylinder #1 (usually the front of the engine on front-wheel-drive vehicles). "Fuel trim" is the adjustment the ECU makes to the amount of fuel being injected into the engine to maintain the ideal air-fuel ratio of 14.7:1.

When the ECU detects that it can no longer compensate for a fuel mixture that is too rich (too much fuel, not enough air) or too lean (too much air, not enough fuel), it sets the P0173 code.

Common symptoms

• Check Engine Light is on
• Poor fuel economy
• Rough idling
• Engine misfires
• Lack of acceleration
• Engine stumbling or hesitation

Common causes

1. Vacuum leak: A leak in a vacuum hose or the intake manifold is the most common cause of P0173. Extra, unmetered air entering the engine will cause a lean condition.
2. Faulty Mass Airflow (MAF) sensor: A dirty or failing MAF sensor can incorrectly measure the amount of air entering the engine, leading to an improper fuel mixture.
3. Clogged fuel filter or failing fuel pump: A restriction in fuel flow can cause a lean condition.
4. Faulty oxygen (O2) sensor: The O2 sensor measures the amount of oxygen in the exhaust, and this data is used to adjust the fuel trim. A faulty sensor can send incorrect data to the ECU.
5. Leaking fuel injector: A leaking fuel injector will cause a rich condition.
6. Faulty fuel pressure regulator: An incorrect fuel pressure can lead to either a rich or lean condition.

How a technician diagnoses it

1. Scan for codes: The technician will first use an OBD-II scanner to confirm the P0173 code and check for any other related codes.
2. Analyze freeze-frame data: This data provides a snapshot of the engine's operating conditions at the time the code was set. This can help to narrow down the potential cause.
3. Check fuel trim values: The technician will look at the short-term and long-term fuel trim values on a scan tool. A positive percentage indicates a lean condition, while a negative percentage indicates a rich condition. Normal values are typically within +/- 10%.
4. Inspect for vacuum leaks: A smoke test is the most effective way to check for vacuum leaks. Smoke is introduced into the intake system, and any leaks will be visible.
5. Test the MAF sensor: The technician will check the MAF sensor's output voltage or frequency to see if it is within the manufacturer's specifications.
6. Check fuel pressure: A fuel pressure gauge will be used to ensure the fuel pressure is within the correct range.
7. Test the oxygen sensors: The technician will check the O2 sensor's operation using a scan tool to see if it is responding correctly to changes in the air-fuel mixture.

Typical repair cost

The cost to repair a P0173 code can vary widely depending on the cause.

• Vacuum leak: Repairing a vacuum leak can be as simple as replacing a cracked hose for under $100, or it could involve a more complex repair like an intake manifold gasket replacement, which could cost several hundred dollars.
• MAF sensor: A new MAF sensor can cost between $150 and $400, depending on the vehicle.
• Fuel filter: A fuel filter replacement is a relatively inexpensive repair, typically costing between $50 and $150.
• Oxygen sensor: An O2 sensor replacement can cost between $100 and $300.
• Fuel pump: A fuel pump replacement is a more expensive repair, typically costing between $400 and $1000.

Can I keep driving with this code?

Yes, you can usually drive with a P0173 code. However, it is not recommended to drive for an extended period of time. A rich or lean condition can cause damage to the catalytic converter, which is a very expensive repair. You will also experience poor fuel economy and engine performance.

Related codes

• P0170: Fuel Trim Malfunction (Bank 1)
• P0171: System Too Lean (Bank 1)
• P0172: System Too Rich (Bank 1)
• P0174: System Too Lean (Bank 2)
• P0175: System Too Rich (Bank 2)

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