P0173 - Fuel Trim System Bank 2 Malfunction OBD-II

P0173 - Fuel Trim System Bank 2 Malfunction (OBD-II)

Definition (source references)

• Code meaning: P0173 is a Fuel Trim (Bank 2) Malfunction. This is a powertrain (OBD-II) code indicating abnormal or out-of-range fuel trim adjustments on the Bank 2 side of the engine. The open-source definition group notes this as "Fuel trim malfunction - bank."
• Context: OBD-II defines many diagnostic trouble codes (DTCs) to monitor engine and emission-related systems. Powertrain codes include fuel trim issues among others. These systems continuously adjust air/fuel delivery to maintain optimal combustion and emissions.

What This Code Means

• Bank 2 is not staying within the expected short-term or long-term fuel trim range. The engine control module (ECM/PCM) has detected a persistent deviation in fuel mixture correction for the Bank 2 bank, suggesting a fuel delivery or air-metering issue (or a sensor/communication fault) affecting that side of the engine.
• This is similar in concept to other P017x codes (e.g., P0171/P0172 for Bank 1), but specific to Bank 2. It is commonly observed with lean or rich indications being driven by adjusted trims, leaks, sensor faults, or fuel delivery problems.

Symptoms

• Check Engine Light (MIL) illuminated or flashing in some vehicles.
• Poor idle quality or rough running on the bank 2 side.
• Hesitation, misfire-like feel, or reduced acceleration especially under load.
• Degraded fuel economy or inconsistent MPG.
• May appear with other DTCs (e.g., P0171/P0174) indicating multi-bank trim concerns, but P0173 can stand alone if Bank 2 is primarily affected.

Probable Causes

Note: Percentages are approximate and reflect typical shop experience rather than a formal NHTSA dataset. They are intended to guide initial troubleshooting priorities.

• Vacuum leaks or unmetered air on Bank 2 intake (manifold, hoses, PCV). 25-40%
• Faulty or dirty MAF sensor or air metering issues on Bank 2, reducing accurate air measurement. 15-30%
• Fuel delivery or pressure issues affecting Bank 2 (weak pump, clogged filter, failing regulator, or clogged injectors). 15-25%
• Faulty or intermittent Bank 2 upstream O2 sensor (O2S Bank 2 Sensor 1) or its heater circuit causing incorrect trims. 10-20%
• Bank 2 injector leakage or sticking on one or more cylinders, causing trim compensation. 5-15%
• Exhaust leaks or EGR/PCV issues that skew sensor readings on Bank 2 before the O2 sensor. 5-10%
• PCM/sensor wiring or connector faults (intermittent or miscommunication). 5-10%

Notes:

• If P0173 appears with P0171 (Bank 1 lean) or P0174 (Bank 2 lean) or P0172/P0175 (rich), multi-bank issues or shared air/fuel delivery pathways may be involved.
• When diagnosing, prioritize root causes that consistently affect bank 2 trims and that would plausibly upset the bank-specific sensors.

Diagnostic Approach

1) Confirm and characterize the fault

• Use a capable scan tool to confirm P0173 and retrieve freeze-frame data.
• Note the bank 2 fuel trim values (short-term and long-term) and the current engine load, RPM, fuel pressure reading (if available), and any other codes.
• Check if P0173 occurs consistently or intermittently across driving conditions (idle, cruise, acceleration). Also check for related codes (P0171, P0172, P0174, P0175, misfire codes, etc.).

2) Baseline comparison

• Compare Bank 2 data to Bank 1 data if possible. If Bank 1 trims are stable and near zero while Bank 2 is drifting, the fault is more likely localized to Bank 2 hardware/sensors rather than a global issue (e.g., MAF or overall fuel pressure)
• If available, view O2 sensor data for Bank 2 Sensor 1 (pre-cat) and Sensor 2 (post-cat) to assess sensor behavior vs. trims.

3) Inspect for obvious issues on Bank 2 side

• Vacuum/air leak inspection: inspect all intake plumbing on Bank 2 side, including manifold gaskets and associated hoses, for leaks, loose connections, or cracks. A smoke test is especially effective to reveal leaks that alter fuel trim.
• MAF sensor: inspect for contamination, dirt, or damage. Consider cleaning or swapping with a known-good unit if readings are suspect.
• Intake cleanliness and restrictions: ensure air filter is clean and that there are no obstructions in the intake tract on the Bank 2 side.
• Check for misrouted or crossed hoses that could affect Bank 2 specifically.

4) Fuel metering and delivery checks

• Fuel pressure test: verify fuel pressure against vehicle specifications with the proper must-have tool. A low pressure could cause lean bank trims; high pressure or intermittent pressure drop can cause odd trims as well.
• Fuel delivery components: inspect the fuel filter, pump, regulator, and basic fuel system integrity. Look for signs of restriction or weakness that could impact Bank 2 fuel delivery.

5) Oxygen sensor and related circuitry

• Check Bank 2 upstream O2 sensor (Sensor 1) operation and heater circuit. Verify the sensor is heating properly and reporting a reasonable switching pattern compared to Bank 1 Sensor 1.
• If Bank 2 Sensor 1 is slow to switch or reads persistently lean/rich relative to Bank 1, consider swapping O2S bank 2 sensor 1 (or testing with a known-good sensor) to determine if the sensor is faulty.
• Inspect harnesses and connectors for Bank 2 O2 sensors for damage or corrosion.

6) Secondary checks: combustion and exhaust side

• Inspect for exhaust leaks before the Bank 2 O2 Sensor 1 location. Leaks can cause erroneous fuel trim readings.
• If applicable, check EVAP system conditions and ensure there is no source of unmetered air or vapor that could affect bank 2 trim values.

7) If apparent faults are not found in the above steps

• Consider sticking or intermittent injector issues on Bank 2 (especially if injectors share a common rail or supply path). Perform injector balance tests if equipment is available.
• Inspect sensor wiring, connectors, and PCM connections for corrosion, damage, or poor grounding. Faulty wiring can mimic or cause fuel trim faults.

8) Post-repair verification

• Clear codes and perform a thorough drive cycle to recheck P0173.
• Verify Bank 2 LTFT and STFT return to near-zero values during idle and steady-state driving under typical operating temperature.
• Confirm that no new codes reappear and that emissions-related monitors complete successfully (as applicable for your vehicle).

Representative tests and troubleshooting tips (practical actions)

• Smoke test the intake for Bank 2 to identify hidden vacuum leaks.
• Measure live fuel pressure while the engine is running and during a load change; compare to factory spec.
• Use a diagnostic tool to log LTFT and STFT for Bank 2 across a full drive cycle; look for persistent drift > ±10-15% over time.
• Swap a suspect Bank 2 upstream O2 sensor with a known-good unit, or swap Bank 2 bank 2 sensor if your tool allows sensor-level testing.
• If bank 2 trims improve after a repair (e.g., vacuum leaks fixed, MAF cleaned, or fuel pressure corrected) and P0173 clears, the repair was successful. If the code persists, continue with deeper diagnostics.

Likely final fixes (typical repair paths)

• Repair vacuum leaks or replace cracked hoses/manifolds on the Bank 2 side.
• Clean or replace a dirty/unresponsive MAF sensor (or adapt new MAF) on Bank 2 side.
• Restore proper fuel pressure (repair fuel pump, replace faulty regulator, replace clogged fuel filter).
• Replace faulty Bank 2 upstream O2 sensor (Sensor 1) or its wiring/heater circuit.
• Repair or replace leaking Bank 2 injectors or address injector wiring/driver issues.
• Repair exhaust leaks before the Bank 2 O2 sensor (including exhaust manifold or downpipe seals).

Safety and operational notes

• Always follow proper fuel system safety procedures when testing or repairing; depressurize the fuel system according to manufacturer guidelines before disconnecting any fuel line components.
• When performing smoke tests, ensure proper ventilation and avoid ignition sources around the engine bay.
• Disconnecting or testing high-voltage or sensor circuits should be performed with appropriate PPE and tools, and only if you're trained to do so.
• After any repair, re-check for stored codes, monitor readiness, and verify the repair under real-world driving conditions.

How to document your findings (for records)

• Record the exact P0173 definition used by your vehicle (Bank 2 fuel trim malfunction).
• Note initial LTFT/STFT values for Bank 2, the engine load, RPM, and temperature from your scan tool.
• List all tests performed, parts replaced, and the observed effect on LTFT/STFT.
• Include a short summary of diagnostic reasoning and final verification results.

References and further reading

• Diagnostic Trouble Codes and overall OBD-II framework: Wikipedia - OBD-II, Diagnostic Trouble Codes; Wikipedia - Powertrain Codes (for context on how DTCs fit within emissions and powertrain monitoring).
• Code-definition reference for P0173 as Fuel Trim (Bank 2) Malfunction. This aligns with the Portuguese entry noted: Mau funcionamento da correção de combustível - banco.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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