P0173
UniversalFuel Trim Malfunction (Bank 2)
Medium SeverityThe engine computer (ECM) has detected that the fuel trim system on Bank 2 is operating outside its normal parameters. This means the engine is either running too rich or too lean on one side of the engine, which can lead to reduced fuel efficiency and potential engine damage over time.
⚠️ Before You Begin (Safety First)
Always ensure the vehicle is parked on a level surface and the engine is off and cool before beginning any inspection or repair. Disconnect the negative battery terminal before working on electrical components. Wear appropriate personal protective equipment, such as gloves and eye protection.
Common Symptoms
- Check Engine Light illuminated
- Reduced fuel economy
- Rough idle or stalling
- Lack of power or poor acceleration
- Engine hesitation or misfires
- Possible sulfur smell from exhaust
Common Causes
- Vacuum leaks (intake manifold, PCV system, brake booster)
- Faulty Mass Air Flow (MAF) sensor
- Malfunctioning oxygen sensor(s) on Bank 2
- Low fuel pressure or clogged fuel filter
- Clogged or faulty fuel injectors on Bank 2
- Exhaust leaks before the oxygen sensor on Bank 2
- Faulty Engine Coolant Temperature (ECT) sensor
- Problem with the Engine Control Module (ECM)
Frequently reported on: All Makes
Diagnostic Steps
- Inspect all vacuum lines and hoses for cracks, disconnections, or leaks, especially around the intake manifold and PCV valve.
- Check the air filter and air intake system for obstructions or damage.
- Carefully remove and clean the Mass Air Flow (MAF) sensor with a specialized MAF sensor cleaner.
- Inspect the electrical connections to the oxygen sensors and MAF sensor for corrosion or damage.
- Consider using an OBD-II scanner to monitor live data for fuel trims (short term and long term) to narrow down if the condition is rich or lean.
👨🔧 When to Call a Professional
If you have checked for obvious vacuum leaks and cleaned the Mass Air Flow (MAF) sensor without success, or if the symptoms are severe (e.g., strong sulfur smell from exhaust, significant loss of power), it is time to consult a professional. Advanced diagnostics with specialized scan tools may be required to pinpoint the exact cause.
Commonly Asked Questions
What does 'Bank 2' mean in P0173?
'Bank 2' refers to the side of the engine that does NOT contain cylinder #1. In V-type or Boxer engines, this indicates an issue specific to one side of the engine's exhaust and fuel system components.
Can I drive with a P0173 code?
While it might be possible to drive short distances, it's not recommended. Prolonged driving with P0173 can lead to reduced fuel economy, increased emissions, and potentially cause damage to expensive components like the catalytic converter due to incorrect fuel mixtures.
How do I test for vacuum leaks?
You can perform a visual inspection of all vacuum lines and connections. A common DIY method is to spray unlit propane or carburetor cleaner around vacuum lines and intake manifold gaskets while the engine is running. If the engine RPM changes, you've found a leak. Be extremely cautious with flammable substances.
Does cleaning the MAF sensor always fix P0173?
Cleaning the Mass Air Flow (MAF) sensor is a common and easy DIY step that often resolves fuel trim issues if the sensor was dirty. However, it's not a guaranteed fix, as many other factors like vacuum leaks or faulty oxygen sensors can also cause P0173.
What is 'fuel trim'?
Fuel trim is the engine computer's adjustment to the amount of fuel injected into the engine to maintain an optimal air-fuel ratio. Short-term fuel trim (STFT) makes immediate adjustments, while long-term fuel trim (LTFT) makes broader, learned adjustments based on driving conditions.