What does the P0173 code mean?
The P0173 is a generic OBD-II powertrain trouble code. Its official definition is “Fuel Trim Malfunction (Bank 2)“.
When you connect a vehicle diagnostics tool to the diagnostic link connector, the powertrain control module (PCM) reports P0173 if it detects that the fuel trim values on Bank 2 have exceeded the maximum allowable adjustment range. Fuel trim is the PCM’s way of compensating for variations in engine conditions by adding or subtracting fuel. Short-term fuel trim (STFT) adjusts rapidly based on oxygen sensor feedback, while long-term fuel trim (LTFT) stores learned corrections. If the total fuel trim (STFT + LTFT) goes beyond a calibrated limit (typically ±20–25%), the PCM determines that the system can no longer compensate properly and sets P0173. Using a professional car scanner with live data allows technicians to view both STFT and LTFT percentages on Bank 2. An obd2 scanner can also capture freeze frame data showing engine load, RPM, and coolant temperature at the moment the fault set. Unlike specific lean or rich codes (P0174, P0175), P0173 is a general fuel trim malfunction that requires diagnosing the underlying cause.
Symptoms of Error Code P0173
- Malfunction Indicator Lamp (MIL) illuminated
- Rough idle or unstable engine speed
- Poor fuel economy (decrease of 2–5 MPG)
- Hesitation or surging during acceleration
- Failed emissions test due to improper air-fuel ratio
- Possible black smoke from exhaust (rich condition) or backfiring (lean condition)
Main Causes of Error Code P0173
- Vacuum leaks on Bank 2 (intake manifold gasket, PCV hoses)
- Faulty mass airflow (MAF) sensor (affects both banks but may show uneven trims)
- Faulty oxygen sensor (Bank 2 Sensor 1)
- Fuel pressure issues (low or high fuel pressure, clogged fuel filter)
- Exhaust leaks before the oxygen sensor on Bank 2
- Malfunctioning fuel injectors on Bank 2 (clogged or leaking)
- Engine mechanical issues on Bank 2 (low compression, worn valves)
- PCM software or calibration errors
How to Diagnose Error Code P0173?
- Connect a vehicle code reader to the DLC port and retrieve all stored DTCs, including freeze frame data. Note the STFT and LTFT values on Bank 2 at idle and at 2500 RPM.
- Perform a thorough visual inspection for vacuum leaks on Bank 2. Listen for hissing sounds, check all vacuum hoses, intake manifold gaskets, and the PCV system.
- Inspect the MAF sensor for contamination. Clean it with MAF cleaner if dirty, or replace if faulty.
- Check fuel pressure using a fuel pressure gauge. Compare with manufacturer specifications (typically 35–65 psi depending on system).
- Monitor oxygen sensor voltages with a car code scanner. Bank 2 Sensor 1 should switch rapidly between 0.1 V and 0.9 V at idle. A lazy or stuck sensor can cause fuel trim issues.
- Perform a smoke test on the intake system to locate hidden vacuum leaks.
- If all else fails, check for exhaust leaks upstream of the oxygen sensor on Bank 2 and inspect fuel injector operation on Bank 2.
- Clear the code with an obd2 scanner after repairs and perform a drive cycle to verify the fix.
Possible Causes and Diagnostic Methods
| Possible Cause | How to Check? |
| Vacuum leak on Bank 2 | Smoke test; listen for hissing; monitor fuel trim response to propane enrichment |
| Faulty MAF sensor | Read MAF grams/second with a car scanner; clean or replace if out of spec |
| Faulty oxygen sensor (Bank 2) | Monitor O2 voltage waveform; check for slow response or stuck voltage |
| Fuel pressure issue | Attach fuel pressure gauge; compare to specs; check regulator and filter |
| Exhaust leak on Bank 2 | Smoke test exhaust; inspect for holes or loose connections before O2 sensor |
| Injector problem on Bank 2 | Balance test; measure resistance; listen for clicking |
Tools needed: obd2 scanner or vehicle diagnostics tool, fuel pressure gauge, smoke machine, digital volt/ohmmeter (DVOM), MAF cleaner, basic hand tools, safety glasses.
How do I fix error code P0173? (Solutions to the Problem)
Simple Fixes
- Clear the code with a car code scanner and drive. If the code does not return, it may have been a temporary anomaly.
- Clean the MAF sensor using approved MAF cleaner.
- Tighten loose hose clamps and reconnect any disconnected vacuum lines.
In-depth Diagnosis and Repair Solutions
- Repair vacuum leaks on Bank 2 by replacing cracked hoses, gaskets, or seals.
- Replace a faulty MAF sensor with an OEM or high-quality aftermarket unit.
- Replace a faulty oxygen sensor (Bank 2 Sensor 1).
- Replace the fuel filter and/or fuel pressure regulator if fuel pressure is out of spec.
- Repair exhaust leaks on Bank 2 by welding or replacing gaskets.
- Clean or replace fuel injectors on Bank 2 if they are clogged or leaking.
- If engine mechanical issues are suspected (compression test), perform necessary engine repairs.
Fix faults based on symptoms
| Observed Symptom | Likely Root Cause | Recommended Action |
| High positive LTFT (+20% or more) on Bank 2 at idle | Vacuum leak on Bank 2 or MAF underreporting | Smoke test Bank 2; clean/replace MAF |
| High negative LTFT (-20% or more) on Bank 2 at idle | Rich condition on Bank 2 (leaky injector, high fuel pressure) | Check fuel pressure; injector balance test on Bank 2 |
| LTFT normal at idle but high under load | Fuel pressure drop or MAF saturation | Test fuel pressure under load; check MAF scaling |
| O2 sensor on Bank 2 stuck lean | Exhaust leak or faulty O2 sensor | Smoke test exhaust; replace O2 sensor |
Common Error Code P0173 in Vehicles
Toyota & Lexus: On Toyota Camry, Corolla, and Lexus ES 350, P0173 often stems from a dirty MAF sensor or vacuum leaks from the intake hose after the air filter. Toyota recommends cleaning the MAF every 60,000 miles. A cracked PCV hose is a common cause on 2.4 L and 3.5 L engines.
Honda & Acura: Honda Accord and Civic with i-VTEC engines frequently experience P0173 due to a faulty idle air control valve or vacuum leaks at the intake manifold gasket on Bank 2. Honda TSB 17-012 advises checking the EGR system for carbon buildup.
Ford & Lincoln: Ford Focus, Fusion, and Lincoln MKZ with 2.0 L and 2.5 L engines may log P0173 when the purge valve sticks open, causing a large vacuum leak. Ford TSB 19-2156 recommends replacing the purge valve and updating PCM software.
General Motors (Chevrolet, Buick, GMC): GM vehicles like Chevy Equinox and Buick Lacrosse with 2.4 L Ecotec engines often see P0173 due to carbon buildup on the intake valves (direct injection). GM TSB PIP5098 recommends walnut blasting the intake valves and updating the PCM calibration.
BMW & Mini: BMW 3 Series and X3 with N52 or N54 engines may set P0173 when the MAF sensor fails or when there is a vacuum leak from the crankcase ventilation system. Replacing the valve cover (integrated PCV) is a common fix on older models.
Volkswagen & Audi: VW Jetta and Audi A4 with 2.0 TSI engines frequently encounter P0173 due to a failing PCV diaphragm or a leaking intake manifold runner flap. VW TSB 01-19-04 recommends replacing the PCV valve and performing an intake cleaning.
P0173 Frequently Asked Questions
Can I drive with a P0173 code?
Yes, but it is not recommended for extended periods. The engine may run poorly, consume more fuel, and potentially damage the catalytic converter. Driving short distances to a repair shop is acceptable.
How much does it cost to fix P0173?
Costs vary widely depending on the root cause. Simple vacuum hose replacement can cost $50–$150. MAF sensor replacement ranges from $150–$400. Oxygen sensor replacement is $100–$300. Fuel injector cleaning or replacement can be $200–$800. Using a vehicle diagnostics tool to pinpoint the exact cause saves money.
Will clearing the code fix P0173?
Clearing the code with an obd2 scanner will turn off the MIL temporarily, but if the underlying problem is not resolved, the code will return. Clearing is only useful after repairs have been made.
Can a bad battery cause P0173?
A weak battery or poor ground connection can cause erratic sensor readings and fuel trim issues, but it is not a common cause. Check battery voltage and ground straps if other causes are ruled out.
How do I reset the P0173 code after repair?
Use an obd2 scanner to clear the code. Then perform a drive cycle that includes idling, steady cruising, and deceleration. The PCM will relearn fuel trims over 10–20 miles of mixed driving. Avoid disconnecting the battery, as that resets adaptive memory and may cause temporary drivability issues.
Is P0173 a serious code?
Moderately serious. It indicates the fuel system on Bank 2 is struggling to maintain correct air-fuel ratio. Left unresolved, it can lead to poor performance, increased emissions, and catalytic converter damage. Early diagnosis with a car scanner is recommended.
P0173 Related OBD2 Errors
- P0170 – Fuel Trim Malfunction (Bank 1)
- P0171 – System Too Lean (Bank 1)
- P0172 – System Too Rich (Bank 1)
- P0174 – System Too Lean (Bank 2)
- P0175 – System Too Rich (Bank 2)
- P0101 – Mass Air Flow Circuit Range/Performance
- P0150 – O2 Sensor Circuit Malfunction (Bank 2 Sensor 1)
- P0151 – O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
- P0152 – O2 Sensor Circuit High Voltage (Bank 2 Sensor 1)
- P0300 – Random/Multiple Cylinder Misfire Detected
Important Warning
Important! Always work in a well-ventilated area when diagnosing fuel system issues. Fuel vapors are flammable. Wear safety glasses and gloves. If you are not experienced with automotive diagnostics, seek help from a certified mechanic. Using a quality obd2 scanner and following manufacturer-specific procedures ensures accurate results and prevents unnecessary part replacements.