OBD Code P0173: Bank 2 Fuel Trim Out of Range – Causes, Symptoms & Fixes
If your vehicle has triggered the P0173 diagnostic trouble code, your engine’s computer has detected that the fuel trim correction on Bank 2 has exceeded acceptable limits. This code is particularly common on V6 and V8 engines, which have two banks of cylinders. Bank 2 is the side of the engine that does not contain cylinder #1. When fuel trim goes out of range, it means the engine’s fuel mixture is either running too rich (too much fuel) or too lean (too little fuel), and the engine computer cannot correct it within normal parameters.
What Does P0173 Mean?
P0173 is a generic OBD-II code that stands for “Fuel Trim Out of Range – Bank 2.” To understand this code, you need to know what fuel trim is and what Bank 2 represents.
Fuel Trim Explained: Modern engines use oxygen sensors to measure how much unburned oxygen is in the exhaust. The engine computer uses this information to adjust the fuel injection duration in real-time, keeping the air-fuel ratio as close to the ideal 14.7:1 ratio as possible. These adjustments are called “fuel trim.” The computer makes two types of adjustments:
- Short-term fuel trim (STFT): Quick, immediate adjustments to fuel delivery
- Long-term fuel trim (LTFT): Slower adjustments that the computer “learns” and remembers
Bank 2: In a V6 or V8 engine, Bank 1 contains cylinder #1, and Bank 2 contains the other cylinders on the opposite side. When P0173 appears, it means the fuel trim on Bank 2 has exceeded the acceptable range (typically ±25% for long-term fuel trim).
When fuel trim goes out of range, it indicates that the engine is running either too rich or too lean, and the computer’s fuel injection adjustments cannot bring it back into the normal operating range. This is a sign that something is wrong with the fuel delivery, air intake, or exhaust monitoring systems.
Common Symptoms
Drivers may notice one or more of the following symptoms when P0173 is present:
- Check Engine Light (CEL): The most obvious symptom; the light will remain on as long as the condition persists
- Rough idle: The engine may run unevenly at a stoplight or in park
- Poor fuel economy: The vehicle may use more gas than usual, especially if running rich
- Black smoke from the exhaust: A sign the engine is running too rich (too much fuel)
- Hesitation during acceleration: The engine may feel sluggish or delayed when you press the gas pedal
- Difficulty starting: The engine may crank longer than normal before firing up
- Fuel smell from exhaust: Unburned fuel in the exhaust creates a strong fuel odor
- Misfires: You may feel a slight shaking or hear popping from the engine or exhaust
- Reduced power: The engine may feel less responsive than normal
Possible Causes (Ranked by Frequency)
P0173 can be caused by several different issues. Here are the most common causes, ranked from most to least likely:
1. Faulty Oxygen Sensor (Bank 2)
This is the most common cause of P0173. The oxygen sensor on Bank 2 may be failing, sending incorrect signals to the engine computer. A degraded sensor can cause the computer to make incorrect fuel trim adjustments. Oxygen sensors typically last 80,000 to 100,000 miles before they need replacement.
2. Vacuum Leak
A vacuum leak allows unmetered air to enter the engine, making the fuel mixture too lean. The engine computer tries to compensate by adding fuel, but if the leak is significant, the fuel trim can exceed its limits. Common sources include cracked hoses, loose fittings, or leaking intake gaskets.
3. Fuel Pressure Regulator Issues
A faulty fuel pressure regulator can cause fuel pressure to be too high or too low. Low fuel pressure results in a lean mixture; high fuel pressure results in a rich mixture. Either condition can push fuel trim out of range.
4. Fuel Injector Problems
A leaking, stuck, or clogged fuel injector on Bank 2 can cause that side of the engine to run rich or lean. If one or more injectors on Bank 2 are not functioning properly, fuel trim will be affected.
5. Mass Airflow (MAF) Sensor Malfunction
The MAF sensor measures the amount of air entering the engine. A dirty or failing MAF sensor sends incorrect readings, causing the computer to miscalculate the fuel injection amount. This affects fuel trim on both banks but may be more pronounced on one side.
6. Air Intake Leak
A crack in the intake manifold or a loose intake hose can allow air to bypass the MAF sensor, causing a lean condition and pushing fuel trim out of range.
7. Faulty Engine Control Module (ECM)
In rare cases, the ECM itself may be faulty and unable to properly calculate or adjust fuel trim. This is typically diagnosed only after all other causes have been ruled out.
8. Fuel Quality Issues
Contaminated or low-quality fuel can affect combustion and fuel trim. Using fuel from a questionable source or old fuel that has degraded can contribute to this code.
Diagnostic Steps
Follow these steps to diagnose the cause of P0173:
Step 1: Retrieve and Document Codes
Use an OBD-II scanner to read all diagnostic trouble codes and freeze frame data. Document the fuel trim values (both short-term and long-term) for Bank 2. Note the conditions when the code was set (engine load, RPM, temperature, etc.).
Step 2: Visual Inspection
Perform a thorough visual inspection of:
- Vacuum hoses for cracks, splits, or loose connections
- Intake manifold gaskets for leaks or damage
- Fuel injector connectors for corrosion or loose connections
- Air intake ducts for cracks or loose clamps
- Oxygen sensor wiring and connectors
Step 3: Smoke Test
If a vacuum leak is suspected, perform a smoke test. A smoke machine will reveal any air leaks in the intake system that might not be visible during a standard inspection.
Step 4: Test the Oxygen Sensor (Bank 2)
Use a digital multimeter or oscilloscope to test the Bank 2 oxygen sensor. The sensor should:
- Show a voltage that oscillates between 0.1V and 0.9V when the engine is running
- Switch between rich and lean conditions multiple times per second
- Have a response time of less than 100 milliseconds
If the sensor voltage is stuck high, stuck low, or not switching, the sensor is faulty and needs replacement.
Step 5: Check Fuel Pressure
Connect a fuel pressure gauge to the fuel rail and measure the fuel pressure at idle and at 2,500 RPM. Compare the readings to the manufacturer’s specifications (typically 44-50 PSI for most vehicles). If the pressure is too high or too low, the fuel pressure regulator may be faulty, or the fuel pump may be failing.
Step 6: Inspect the MAF Sensor
Remove the MAF sensor and inspect it for dirt, oil, or contamination. The sensor element is very delicate and should never be touched. If dirty, clean it with MAF sensor cleaner. If the sensor is damaged or cannot be cleaned, it should be replaced.
Step 7: Check Fuel Injector Operation
Use a fuel injector tester to verify that all fuel injectors on Bank 2 are firing correctly and delivering fuel evenly. Listen for a clicking sound when the injector is energized, and observe fuel spray pattern if possible.
Step 8: Perform a Fuel Trim Relearn
After addressing the suspected cause, clear the code and allow the engine to relearn fuel trim values. Drive the vehicle under various conditions (idle, acceleration, highway speeds) for at least 50 miles. Then scan the vehicle again to see if the code returns.
Repair Cost Estimates
The cost to repair P0173 varies depending on the underlying cause:
- Oxygen Sensor Replacement: $150–$400 (parts and labor). This is the most common repair.
- Vacuum Leak Repair: $100–$500, depending on the location and severity of the leak.
- Fuel Pressure Regulator Replacement: $200–$600.
- Fuel Injector Replacement: $300–$900 per injector, depending on the vehicle.
- MAF Sensor Replacement: $200–$500.
- Intake Manifold Gasket Replacement: $300–$800.
- ECM Reprogramming or Replacement: $500–$1,500+ (rare).
The most cost-effective approach is to start with the most common causes (oxygen sensor and vacuum leak inspection) before moving on to more expensive repairs.
Can I Still Drive?
P0173 is a moderate severity code. In most cases, you can continue driving, but with caution:
- Safe to drive: If the only symptom is the Check Engine Light and the engine runs normally, you can drive the vehicle to a mechanic for diagnosis and repair.
- Use caution: If you notice rough idle, hesitation, or poor fuel economy, the engine may be running inefficiently. Continued driving could damage the catalytic converter, especially if the engine is running too rich.
- Do not ignore: While not an emergency, P0173 should be diagnosed and repaired within a few days to avoid more expensive damage.
- Catalytic converter risk: If the engine is running too rich for an extended period, unburned fuel can damage the catalytic converter, which is a very expensive repair ($500–$2,500).
It is recommended to have the code diagnosed and repaired as soon as possible to prevent further damage and maintain fuel efficiency.
Frequently Asked Questions
Q: What is the difference between Bank 1 and Bank 2?
A: Bank 1 is the side of the engine that contains cylinder #1. Bank 2 is the opposite side. In a V6 or V8 engine, the banks are on opposite sides of the engine block. Some inline engines may have banks divided by firing order rather than physical location. You can find which side is Bank 2 in your vehicle’s service manual or by checking the oxygen sensor location.
Q: Can a bad oxygen sensor cause P0173?
A: Yes, absolutely. A faulty oxygen sensor on Bank 2 is the most common cause of P0173. The sensor may be stuck at a high voltage, stuck at a low voltage, or responding too slowly. Any of these conditions will cause the engine computer to make incorrect fuel trim adjustments, pushing the trim out of range.
Q: Will P0173 go away on its own?
A: No, P0173 will not go away on its own unless the underlying problem is fixed. The code will remain stored in the engine computer’s memory until the issue is resolved and the code is cleared. Even after clearing the code, it will return if the problem persists.
Q: Can a vacuum leak cause P0173?
A: Yes, a vacuum leak is the second most common cause of P0173. A leak allows unmetered air to enter the engine, making the fuel mixture too lean. The engine computer tries to compensate by adding fuel, but if the leak is large enough, the fuel trim will exceed its limits. A smoke test is the best way to find a vacuum leak.
Q: Is P0173 the same as P0172?
A: No, they are related but different codes. P0172 means “Fuel System Too Rich – Bank 2,” indicating the fuel mixture is too rich. P0173 means “Fuel Trim Out of Range – Bank 2,” indicating the fuel trim has exceeded acceptable limits (which could be rich or lean). P0173 is more general and can indicate either a rich or lean condition, while P0172 specifically indicates a rich condition.