P0173
Fix within a few daysFuel Trim Malfunction, Bank 2
Code P0173 means your engine's computer is struggling to maintain the correct fuel mixture on Bank 2. Common causes: Vacuum leaks (35%), MAF sensor (25%), O2 sensor (20%). Drivability: Yes, for short trips only. Repair cost: $50-$200 DIY, $150-$500 professional.
Can I Drive With P0173?
Yes, for short trips only
Essential trips under 10 miles, avoid highways and heavy acceleration. Fix within a few days.
Stop driving immediately if: Flashing Check Engine Light, significant power loss, loud engine noises, strong fuel smell, or rotten egg smell from exhaust.
Symptoms of P0173
- 1Check Engine Light is on
- 2Rough idle or engine hesitation
- 3Reduced engine power or poor acceleration
- 4Decreased fuel economy
- 5Engine stalling (less common)
Common Causes of P0173
Parts most likely causing this code, ranked by probability
Unmetered air entering the engine on Bank 2 causes the computer to incorrectly adjust fuel delivery.
35% of P0173 cases are fixed by repairing a vacuum leak
A dirty or faulty MAF sensor provides incorrect air intake readings, leading to improper fuel mixture calculations.
25% of P0173 cases are fixed by replacing or cleaning the MAF sensor
A failing upstream O2 sensor on Bank 2 sends inaccurate exhaust gas readings to the computer, causing incorrect fuel trim adjustments.
20% of P0173 cases are fixed by replacing the Bank 2 upstream O2 sensor
Clogged or faulty fuel injectors on Bank 2 deliver too much or too little fuel, forcing the computer to make extreme fuel trim corrections.
10% of P0173 cases are fixed by servicing or replacing fuel injectors
An exhaust leak before the upstream O2 sensor on Bank 2 allows outside air to enter, skewing the O2 sensor's readings.
5% of P0173 cases are fixed by repairing an exhaust leak
A faulty fuel pressure regulator can cause inconsistent fuel pressure, leading to an incorrect fuel mixture that the computer struggles to correct.
5% of P0173 cases are fixed by replacing the fuel pressure regulator
Check These First (Before Buying Parts)
- Visually inspect all vacuum hoses and connections on Bank 2 for cracks, disconnections, or signs of wear.
- Check the air filter to ensure it's clean and properly seated.
- Clean the Mass Air Flow (MAF) sensor with a dedicated MAF sensor cleaner.
How to Diagnose P0173
- 1Connect an OBD-II scan tool and check for other related codes.
- 2Inspect all vacuum lines and intake manifold gaskets on Bank 2 for cracks, loose connections, or signs of leaks.
- 3Visually inspect the Mass Air Flow (MAF) sensor for dirt or debris, and clean it with MAF sensor cleaner.
- 4Use a scan tool to monitor live data for the Bank 2 upstream O2 sensor and fuel trim values (Short Term Fuel Trim - STFT and Long Term Fuel Trim - LTFT).
- 5Check fuel pressure at the fuel rail using a fuel pressure gauge.
- 6Perform a smoke test to pinpoint any hard-to-find vacuum or exhaust leaks on Bank 2.
- 7Test fuel injector resistance and spray pattern on Bank 2 if other causes are ruled out.
P0173 on Popular Vehicles
Mercedes-Benz
2000-2015Commonly linked to vacuum leaks from the intake manifold, PCV system hoses, or a failing Mass Air Flow (MAF) sensor. These engines are sensitive to unmetered air.
60% fixed by repairing vacuum leaks or replacing MAF sensor
BMW
2000-2016Often points to issues with the crankcase ventilation (CCV) system, intake boots, or a faulty MAF sensor. BMW engines are particular about air intake integrity.
55% fixed by repairing vacuum leaks or replacing MAF sensor
Nissan/Infiniti
2003-2020Frequently caused by a dirty or failing Mass Air Flow (MAF) sensor, or sometimes a faulty upstream O2 sensor on Bank 2. Vacuum leaks are also possible but less common than MAF issues.
50% fixed by replacing MAF or O2 sensor
Frequently Asked Questions
What does P0173 mean?
P0173 indicates that your vehicle's engine computer (PCM) has detected an issue with the fuel trim on Bank 2, meaning it's making extreme adjustments to compensate for a fuel mixture that is either too rich or too lean.
Can I drive with P0173?
Yes, for short trips only. You can drive your vehicle with a P0173 code, but you should limit driving and fix it within a few days. Prolonged driving with an incorrect fuel mixture can lead to reduced fuel economy, poor engine performance, and potentially more expensive damage to your catalytic converter.
Will P0173 clear itself?
No, P0173 will not clear itself. The Check Engine Light will remain on until the underlying issue causing the fuel trim malfunction is diagnosed and repaired. After the repair, you can clear the code with a scan tool, or it will eventually clear after several drive cycles if the problem is resolved.
How do I fix P0173 myself?
Start by checking for obvious vacuum leaks around the intake manifold and vacuum hoses on Bank 2. Clean your Mass Air Flow (MAF) sensor with MAF cleaner. If those don't work, you'll need a scan tool to monitor O2 sensor readings and fuel trims to pinpoint if it's a faulty O2 sensor or another component like a fuel injector.
How much does it cost to fix P0173?
The cost to fix P0173 varies. DIY repairs can range from $50 (for MAF cleaner or a vacuum hose) to $200 (for an O2 sensor or intake manifold gasket). Professional shop repairs typically cost between $150 and $500, depending on the specific cause and labor rates.
P0173 keeps coming back - why?
If P0173 keeps returning, it means the root cause was not fully addressed. Common reasons for recurrence include an overlooked vacuum leak, a misdiagnosed MAF or O2 sensor, or an underlying issue like a failing fuel injector or exhaust leak that was not initially identified.
Will P0173 fail emissions/smog test?
Yes, P0173 will cause your vehicle to fail an emissions or smog test. The Check Engine Light being on is an automatic failure in most jurisdictions, and the underlying fuel trim issue means your vehicle is not operating within acceptable emissions parameters.
What's the difference between P0173 and P0171/P0174?
P0173 is a general fuel trim malfunction code for Bank 2, indicating the computer is struggling to correct the fuel mixture. P0171 (Bank 1) and P0174 (Bank 2) specifically indicate a 'System Too Lean' condition, meaning there's too much air or not enough fuel. P0173 is a broader code that can encompass both lean and rich conditions.
P0173 on Mercedes-Benz
On Mercedes-Benz vehicles, P0173 is frequently caused by vacuum leaks, especially from the intake manifold gaskets, crankcase ventilation system hoses, or a faulty Mass Air Flow (MAF) sensor. These components are common failure points on many Mercedes models.
Questions to Ask Your Mechanic
Did you check for vacuum leaks on Bank 2 using a smoke machine?
Good answer: Yes, we performed a smoke test and found no leaks, or we found and repaired a leak at [specific location].
What are the live fuel trim readings (STFT and LTFT) for Bank 2?
Good answer: The short-term fuel trim for Bank 2 is [X]% and the long-term fuel trim is [Y]%. They were out of spec, but are now within +/- 10% after the repair.
Did you test the Mass Air Flow (MAF) sensor's readings?
Good answer: Yes, we checked the MAF sensor's voltage/frequency at idle and under load, and it was either out of specification or within spec after cleaning/replacement.
Repair Cost Estimate
DIY Repair
Parts only
$50-$200
Shop Repair
Parts + labor
$150-$500
Save $100-$300 by doing it yourself
Difficulty
intermediate
Time
1-3 hours
Intermediate - requires a scan tool for proper diagnosis and potentially specific tools for fuel pressure testing or O2 sensor replacement.
Cost of Waiting
Ignoring P0173 can lead to significantly reduced fuel economy and poor engine performance. If the fuel mixture is consistently too rich or too lean, it can cause premature failure of your catalytic converter ($800-$2,500 repair) or damage to engine components like spark plugs and O2 sensors.
While You're In There...
If you're cleaning or replacing the MAF sensor, it's a good time to replace the air filter to ensure clean air flow and prevent future MAF contamination.
If you find a vacuum leak at the intake manifold, replacing the gasket is necessary. It's a common failure point and replacing it prevents repeat labor.
Tools You'll Need
- OBD-II Scan Tool (with live data capability)
- Basic Hand Tools (sockets, wrenches, screwdrivers)
- MAF Sensor Cleaner
- Vacuum Gauge (optional, but helpful)
- Fuel Pressure Gauge (optional, but helpful)
- Smoke Machine (for advanced leak detection, can be rented)
P0173 is a common fuel system code. Most repairs are straightforward, and once fixed, your engine will run more efficiently and reliably.
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Your Next Steps
Step 1: Perform a thorough visual inspection for vacuum leaks around the intake manifold and all vacuum lines on Bank 2. Clean the MAF sensor.
Step 2: If no obvious leaks are found and cleaning the MAF doesn't help, use an OBD-II scan tool to monitor live data. Pay close attention to Bank 2 upstream O2 sensor readings and fuel trim values (STFT and LTFT).
Step 3: Based on scan tool data, if O2 sensor readings are erratic or fuel trims are consistently high/low, consider replacing the Bank 2 upstream O2 sensor. If fuel pressure is low, investigate the fuel pump or regulator. If fuel trims are still off after addressing these, consider fuel injector testing/replacement for Bank 2.