P0173 – Fuel Trim (Bank 2) indicates that the engine control unit has detected abnormal fuel trim adjustments on Bank 2. Fuel trim describes how much the ECU must correct the air-fuel mixture to keep combustion stable. When the correction becomes excessive—either too rich or too lean—the ECU stores P0173. This may cause rough idle, hesitation, poor fuel economy, or strong exhaust odors. This guide explains what P0173 means, its symptoms, causes, diagnosis steps, and the most effective repair solutions.
What Does P0173 Mean?
P0173 is a generic OBD-II trouble code that means the ECU is unable to maintain proper air-fuel mixture on Bank 2 using normal fuel trim adjustments. Instead of making fine corrections, the ECU is pushed to its limits due to vacuum leaks, fuel system issues, incorrect sensor feedback, or airflow problems.
This fault typically appears on V6 and V8 engines where Bank 2 contains the cylinder bank opposite Bank 1. If only one bank shows trim imbalance, the issue is usually localized to that side of the engine.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Fuel Trim / Air-Fuel Control
- Difficulty Level: Moderate
- Estimated Repair Cost: €90–€350+
- Last Updated: 10 December 2025
Real-World Example / Field Notes
A V6 sedan arrived with noticeable hesitation during acceleration. Live data showed LTFT on Bank 2 at +20%, indicating a lean condition. Smoke testing revealed a cracked vacuum hose feeding Bank 2’s intake runner. After replacing the hose, trims normalized and the fault did not return.
Another case involved a V8 SUV running rich on Bank 2. The upstream oxygen sensor (Bank 2 Sensor 1) was slow-switching and biased rich. Replacing the O2 sensor restored normal trim behavior and improved fuel economy.
Symptoms of P0173 – Fuel Trim (Bank 2)
- Poor fuel economy: Excessive trim correction increases fuel use.
- Rough idle: Air-fuel imbalance causes unstable combustion.
- Hesitation or stumble: Especially noticeable during throttle changes.
- Strong exhaust odor: If Bank 2 is running rich.
- Pinging or detonation: If Bank 2 is running lean.
- Check engine light: Stored when trim limits are exceeded.
Common Causes of P0173
Most Common Causes
- Vacuum leaks affecting Bank 2 intake runners
- Faulty mass airflow sensor (MAF)
- Failing Bank 2 upstream O2 sensor (B2S1)
- Fuel pressure too high or too low
- Leaking or clogged injectors on Bank 2
Less Common Causes
- PCV valve failure
- Exhaust leaks before the B2S1 O2 sensor
- Boost system leaks on turbocharged engines
- Faulty barometric or intake air temperature sensor
- ECU calibration issues
Diagnosis: Step-By-Step Guide
Diagnosing P0173 requires checking Bank 2–specific airflow, fuel delivery, and sensor readings to determine why the ECU cannot maintain proper trim.
Tools You’ll Need
OBD-II scanner with live data, smoke machine, multimeter, fuel pressure gauge, and basic tools.
- Check fuel trim values: Compare STFT and LTFT for both banks.
- Smoke-test the intake system: Look for leaks in hoses, gaskets, PCV lines, and Bank 2 runners.
- Inspect the MAF sensor: Check readings and clean or replace if contaminated.
- Test the Bank 2 upstream O2 sensor: Ensure it switches quickly and accurately.
- Measure fuel pressure: Confirm pump and regulator functionality.
- Check injectors on Bank 2: Look for clogging or leaking nozzles.
- Inspect intake ducting and air filter: Any blockage affects airflow measurement.
- Check for exhaust leaks: Leaks before B2S1 cause false readings.
- Review turbo/wastegate function (if equipped): Leaks create lean trims on Bank 2.
- Inspect PCV system: Faulty valves introduce unmetered air.
Pro Tip: Compare Bank 1 and Bank 2 trim values. If Bank 2 trims are abnormal while Bank 1 is normal, the issue is almost certainly localized to Bank 2 airflow or exhaust before the O2 sensor.
Possible Fixes & Repair Costs
- Fix vacuum leaks: €20–€150
- Replace MAF sensor: €80–€200
- Replace Bank 2 Sensor 1 O2 sensor: €70–€180
- Clean or replace injectors (Bank 2): €60–€250
- Repair fuel pump or regulator issues: €120–€350+
- Repair intake or exhaust leaks: €40–€150
Can I Still Drive With P0173?
Driving is possible, but caution is advised. A lean condition on Bank 2 can cause pinging and overheating, while a rich condition may damage the catalytic converter. Avoid heavy acceleration until the cause is identified and repaired.
Need wiring diagrams and factory-style repair steps?
Powertrain faults often require exact wiring diagrams, connector pinouts, and guided test steps. A repair manual can help you confirm the cause before replacing parts.
Related Fuel Trim Codes
Compare nearby fuel trim trouble codes with similar definitions, fault patterns, and diagnostic paths.
- P0170 – Fuel Trim Bank 1
- P2099 – Post Catalyst Fuel Trim System Too Rich Bank 2
- P2098 – Post Catalyst Fuel Trim System Too Lean Bank 2
- P2097 – Post Catalyst Fuel Trim System Too Rich Bank 1
- P2096 – Post Catalyst Fuel Trim System Too Lean Bank 1
- P0169 – Incorrect Fuel Composition
Key Takeaways
- P0173 indicates abnormal or excessive fuel trim correction on Bank 2.
- Vacuum leaks, MAF sensor issues, and faulty O2 sensors are top causes.
- Driving is possible but may cause engine or catalyst damage if ignored.
- A smoke test and live fuel trim comparison are the fastest diagnosis tools.
FAQ
Is P0173 always caused by a vacuum leak?
No, although vacuum leaks are common, faulty MAF sensors and O2 sensors frequently cause P0173 as well.
Does P0173 affect performance?
Yes. Incorrect trims lead to hesitation, rough running, and poor fuel economy.
Can bad injectors cause P0173?
Yes. Leaking or clogged injectors on Bank 2 can create rich or lean trim conditions.
Should I replace the MAF sensor automatically?
No. Test it first. A contaminated MAF can be cleaned, and other faults may mimic MAF issues.
Can the code clear itself?
If trims normalize, the ECU may clear the code after several drive cycles, but the root cause must still be repaired.