P0173 Code: Fuel Trim Malfunction Bank 2
P0173 is an OBD-II fault code that means Fuel Trim Malfunction Bank 2. The engine control module (ECM/PCM) has detected that the fuel correction on Bank 2 has moved outside the expected operating range.
The exact meaning and diagnostic strategy can vary between manufacturers. P0173 may be encountered on vehicles from BMW, Mercedes-Benz, Nissan, Toyota, Mitsubishi, Hyundai, Jaguar, and Range Rover, among others.
Fault Code Description
P0173 = Fuel Trim Malfunction Bank 2.
The ECM continuously adjusts fuel delivery using information from the MAF/MAP system and oxygen or air-fuel ratio sensors. If the required correction on Bank 2 becomes excessive or remains outside the expected range, P0173 may be stored.
Symptoms of P0173
- Check Engine Light.
- Rough or unstable idle.
- Poor acceleration.
- Hesitation during driving.
- Increased fuel consumption.
- Hard starting in some cases.
- Engine may run too lean or too rich.
Common Causes of P0173
- Incorrect MAF sensor readings.
- Vacuum or intake air leaks.
- Faulty oxygen or air-fuel ratio sensor.
- Incorrect fuel pressure.
- Restricted fuel delivery.
- Dirty or malfunctioning fuel injectors.
- Exhaust leak before the oxygen sensor.
- PCV or crankcase ventilation leaks.
- Fuel contamination.
- Damaged wiring or electrical connections.
How to Fix P0173
1. Check Fuel Trim Data
Use a scan tool to monitor STFT and LTFT for Bank 1 and Bank 2. Comparing both banks can help determine whether the problem is isolated to Bank 2 or caused by a component affecting the entire engine.
2. Inspect for Intake and Vacuum Leaks
Check vacuum hoses, intake boots, PCV connections and the intake manifold. A smoke test can help locate leaks that are difficult to see visually.
3. Check the MAF Sensor
An inaccurate MAF reading can cause the ECM to calculate incorrect fuel delivery. Check the sensor readings against the engine's operating conditions before replacing it.
4. Check Fuel Pressure
Verify fuel pressure against the manufacturer's specification. Low pressure can create a lean condition, while excessive pressure can contribute to a rich condition.
5. Check Oxygen or Air-Fuel Ratio Sensors
Check the Bank 2 sensor response and wiring. Do not automatically replace the oxygen sensor simply because P0173 is present; the sensor may be correctly reporting a mixture problem caused by another component.
6. Check Injectors
Inspect Bank 2 fuel injectors for restricted flow, leakage or electrical problems. Injector imbalance can produce abnormal fuel-trim corrections between the two engine banks.
7. Check for Exhaust Leaks
An exhaust leak before the oxygen or air-fuel ratio sensor can introduce outside air into the exhaust stream and cause incorrect mixture feedback.
P0173 and P0170
P0170 is generally associated with a fuel-trim malfunction on Bank 1 or a general fuel-trim monitoring strategy, while P0173 specifically refers to Bank 2 on applications that use the bank designation.
If P0170 and P0173 appear together, compare the fuel-trim values on both banks. A problem affecting both banks may point toward a common cause such as fuel pressure, MAF measurement, unmetered air or fuel-quality problems.
P0173 and P0172
P0172 indicates that the system is too rich on Bank 1, while P0173 indicates a fuel-trim malfunction on Bank 2. When both codes occur together, compare Bank 1 and Bank 2 fuel-trim readings before replacing sensors or injectors.
Manufacturer-Specific Information for P0173
P0173 BMW
On BMW vehicles, investigate unmetered air, crankcase ventilation leaks, MAF readings, fuel delivery and oxygen-sensor feedback. The engine and fuel-control system determine the exact diagnostic procedure.
P0173 Mercedes-Benz
Mercedes-Benz applications can require checking mixture adaptation values, MAF readings, intake leaks, fuel pressure and oxygen-sensor feedback. Use vehicle-specific specifications when evaluating fuel-trim limits.
P0173 Nissan
On Nissan vehicles, check fuel-trim data together with the MAF sensor, intake system, fuel pressure, injectors and heated oxygen sensor operation. The exact monitoring strategy depends on the engine.
P0173 Toyota
Toyota diagnosis should include comparison of fuel-trim values, MAF/MAP information where applicable, air-fuel ratio sensor operation, fuel pressure and possible intake leaks.
P0173 Mitsubishi
For Mitsubishi vehicles, inspect airflow measurement, fuel delivery, intake leaks and oxygen or air-fuel ratio sensor operation. Engine-specific specifications should be used during testing.
P0173 Hyundai
Hyundai applications should be checked for intake leaks, incorrect airflow readings, fuel-pressure problems, injector issues and abnormal oxygen or air-fuel ratio sensor feedback.
P0173 Jaguar
On Jaguar vehicles, check fuel-trim adaptation together with MAF readings, intake and crankcase ventilation leaks, fuel pressure and oxygen-sensor feedback. The diagnostic procedure varies by engine.
P0173 Range Rover
On Range Rover applications, P0173 should be diagnosed using live fuel-trim data rather than replacing a component based only on the code. Check air measurement, intake leaks, fuel delivery and Bank 2 sensor feedback.
P0173 Diagnostic Signal Path
Repair Cost for P0173
- Vacuum or intake leak: approximately $50–$250+.
- MAF sensor: approximately $100–$400+.
- Oxygen/A-F sensor: approximately $100–$400+.
- Injector or fuel-system repair: varies considerably by vehicle.
These are general estimates. The actual repair cost depends on the vehicle, engine, failed component, parts prices and labor rate.
Common Repair Mistakes
- Replacing the oxygen sensor without checking fuel-trim data.
- Ignoring vacuum and intake leaks.
- Replacing the MAF sensor without verifying its readings.
- Ignoring fuel pressure.
- Replacing injectors without testing their operation.
- Diagnosing the code without comparing Bank 1 and Bank 2 data.
Is P0173 Serious?
P0173 should not be ignored. The vehicle may continue to run, but an unresolved fuel-control problem can cause poor fuel economy, drivability problems, increased emissions and, in some cases, damage to the catalytic converter if the mixture remains excessively rich.
Can I Drive With P0173?
If the engine runs normally and the Check Engine Light is steady, limited driving may be possible. However, the cause should be diagnosed as soon as possible. If the engine is running very poorly, misfiring, losing power or the Check Engine Light is flashing, continued driving should be avoided.
FAQ About P0173
What does P0173 mean?
P0173 means Fuel Trim Malfunction Bank 2. The ECM has detected that fuel correction on Bank 2 is outside its expected range.
How do I fix P0173?
Start by checking STFT and LTFT, then inspect for intake leaks, MAF errors, fuel-pressure problems, injector problems and abnormal Bank 2 oxygen or air-fuel sensor feedback.
What is the most common cause of P0173?
There is no single cause for every vehicle. Common possibilities include incorrect airflow measurement, unmetered air, fuel-delivery problems and inaccurate oxygen or air-fuel sensor feedback.
Can a bad MAF sensor cause P0173?
Yes. An incorrect MAF signal can cause the ECM to calculate incorrect engine airflow and make excessive fuel corrections.
Can a vacuum leak cause P0173?
Yes. Unmetered air entering the engine can cause the ECM to increase fuel delivery and eventually trigger a fuel-trim fault.
What is the difference between P0172 and P0173?
P0172 indicates a system too rich condition on Bank 1, while P0173 indicates a fuel-trim malfunction on Bank 2.
What is the difference between P0170 and P0173?
P0170 is a fuel-trim malfunction code commonly associated with Bank 1 or the vehicle's general fuel-trim monitoring strategy, while P0173 is specifically associated with Bank 2 on applicable vehicles.
Related Codes
- P0170 – Fuel Trim Malfunction
- P0171 – System Too Lean Bank 1
- P0172 – System Too Rich Bank 1
- P0174 – System Too Lean Bank 2
- P0175 – System Too Rich Bank 2
Final Result
P0173 indicates a fuel-trim malfunction involving Bank 2. The code does not automatically mean that the oxygen sensor or MAF sensor is defective. The most reliable approach is to examine fuel-trim data first and then test the intake system, airflow measurement, fuel pressure, injectors and Bank 2 sensor feedback.
Because P0173 can behave differently between BMW, Mercedes-Benz, Nissan, Toyota, Mitsubishi, Hyundai, Jaguar and Range Rover, always use the specifications and diagnostic procedure for the exact engine and vehicle.