P0173 – Fuel Trim Bank 2

Last updated: April 4, 2026

DTC P0173 is an ISO/SAE controlled, general powertrain code with the official title P0173 – Fuel Trim Bank 2 and the official meaning Fuel Trim Bank 2. In practical terms, it indicates the engine control module (ECM/PCM) has identified an abnormal fuel-trim condition specifically affecting Bank 2 (the cylinder bank that does not contain cylinder #1). Fuel trim is the computer’s ongoing adjustment to injector on-time to keep the air-fuel mixture within the target range for stable combustion and emissions control. When Bank 2 corrections become abnormal compared with expected behavior, the ECM stores P0173 and may illuminate the check engine light. Because many faults can influence fuel trim, accurate diagnosis relies on data and testing rather than parts replacement.

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P0173 Quick Answer

P0173 – Fuel Trim Bank 2 (Fuel Trim Bank 2) means the ECM has detected an abnormal fuel-trim condition on Bank 2. Start by confirming Bank 2 short- and long-term fuel trims in live data, then check for intake air leaks, exhaust leaks upstream of the Bank 2 upstream oxygen sensor, and fuel delivery or injector issues that could force the ECM to make excessive corrections on Bank 2.

What Does P0173 Mean?

P0173 is defined as Fuel Trim Bank 2. Fuel trim is the ECM’s calculated correction to commanded fueling based on sensor feedback and operating conditions. When P0173 sets, the ECM has determined that the fuel-trim behavior on Bank 2 is abnormal relative to what it expects for the current engine state (such as idle, cruise, or moderate load) and control strategy.

This code does not, by itself, identify a failed part. It identifies a fuel-trim control issue on Bank 2, which can be driven by unmetered air entering the engine, inaccurate airflow/load inputs, exhaust leaks that skew feedback, or fuel delivery and injector problems that create an actual mixture error on Bank 2. The most productive approach is to use freeze-frame and live data to determine when and how Bank 2 trim becomes abnormal, then test systems that can influence that bank.

Theory of Operation

The ECM calculates a base injector pulse width using engine speed, airflow/load information, and fuel system models. Once conditions allow closed-loop operation, the ECM uses feedback from the upstream oxygen sensor (or air-fuel ratio sensor, depending on design) to adjust fueling. Those corrections appear as short-term fuel trim (STFT)—quick, moment-to-moment changes—and long-term fuel trim (LTFT)—learned adjustments stored over time to correct persistent bias.

Bank-specific fuel trim exists because each cylinder bank can experience different airflow distribution, different exhaust feedback, and different injector behavior. If Bank 2 receives extra unmetered air, has an exhaust leak near its sensor, or suffers from restricted fuel delivery to its injectors, the ECM will attempt to compensate by changing fuel trim. When the correction pattern on Bank 2 becomes abnormal for long enough under the enabling conditions for the monitor, the ECM records P0173 to indicate that Bank 2 fuel trim control is not behaving as expected.

Symptoms

• Check engine light illuminated (MIL on)
• Rough idle, especially as engine load changes (gear engagement, electrical load, A/C load)
• Hesitation, stumble, or surge during acceleration
• Reduced fuel economy
• Hard starting or unstable idle after warm restart (depending on the underlying cause)
• Possible fuel odor from the exhaust if the mixture is driven rich by corrections
• Additional DTCs may be stored, such as misfire codes, oxygen sensor-related codes, or other fuel/air metering codes

Common Causes

• Intake system air leak affecting Bank 2 (vacuum hose leak, PCV-related leak, intake manifold gasket leak localized to Bank 2 runners, or intake duct leak after the MAF sensor)
• Exhaust leak upstream of the Bank 2 upstream oxygen sensor (introducing outside air and altering sensor feedback)
• Mass airflow (MAF) sensor contamination or incorrect airflow signal (including wiring/connector issues affecting the MAF circuit)
• Bank 2 upstream oxygen sensor signal problems (slow response, bias, or circuit faults that mislead closed-loop control)
• Fuel delivery problems (low fuel pressure, restricted supply, or pressure regulation issues affecting mixture control)
• Injector problems that affect Bank 2 (restricted flow, imbalance, or leakage contributing to a bank-specific mixture error)
• EVAP purge flow faults (purge valve leaking when not commanded, plumbing faults altering mixture contribution)
• Electrical wiring/connector issues affecting sensors used for fueling calculation or feedback (corrosion, poor pin fit, damaged harness)

Diagnosis Steps

Tools you’ll need: A scan tool capable of reading live data and freeze-frame data (including STFT/LTFT for both banks), a digital multimeter (DMM), and a smoke machine or other method to check for intake leaks. A fuel pressure gauge is strongly recommended to verify supply. If available, use scan tool functions for O2 sensor graphing and EVAP purge command.

1. Confirm the code and capture data. Verify P0173 is present (pending, confirmed, or history) and save freeze-frame data. Note coolant temperature, RPM, load, vehicle speed, fuel system status (open/closed loop), and STFT/LTFT for Bank 2. Compare Bank 2 trims to Bank 1 at the same moment.
2. Check for other stored DTCs first. Record all codes and address any that directly affect airflow measurement, oxygen sensor operation, misfire detection, or fuel pressure control. Those can change the fuel-trim picture and can invalidate conclusions if ignored.
3. Verify the basics: air intake integrity. Inspect the air filter housing, intake ducting, and clamps from the airbox through the throttle body. Pay close attention to cracks or loose connections after the MAF sensor, because post-MAF leaks create unmetered air and can drive fuel trim abnormal.
4. Review fuel trims at idle and at a steady higher RPM. With the engine warmed up and in closed loop, observe Bank 1 and Bank 2 STFT/LTFT at idle, then again at a steady 2,000–2,500 RPM. A pattern that is worse at idle and improves with RPM often points toward an intake/vacuum leak; a pattern that worsens with load can suggest fuel delivery limitations or sensor/load calculation errors.
5. Check for intake leaks with a smoke test. Smoke-test the intake system (including PCV and vacuum circuits). If the intake manifold has separate runners per bank, focus on potential leak points that could preferentially affect Bank 2. Repair any verified leak before continuing.
6. Evaluate EVAP purge influence. Using the scan tool, observe purge command and (if supported) purge flow/EVAP data. Temporarily command purge off (when safe and supported) and watch whether Bank 2 trims move toward normal. If trims change significantly when purge is disabled, inspect purge valve sealing and purge plumbing for faults.
7. Inspect for exhaust leaks ahead of the Bank 2 upstream sensor. Check the exhaust manifold, gaskets, fasteners, and any joints upstream of the Bank 2 upstream oxygen sensor for leakage (soot traces, ticking sounds, visible cracks). Repair confirmed leaks and recheck trims.
8. Assess Bank 2 upstream oxygen sensor performance. With a graphing view, verify the upstream sensor responds to throttle changes and closed-loop transitions. If response is slow, stuck, or erratic, inspect the sensor wiring and connector (heater power/ground and signal integrity) before condemning the sensor itself.
9. Verify fuel pressure and fuel delivery under load. Measure fuel pressure according to service information and observe whether pressure remains stable during snap throttle and under driving load. A pressure drop can cause a true lean condition that forces high fuel trim correction.
10. Check for bank-specific fueling issues. If Bank 2 trims are abnormal while Bank 1 stays relatively normal, consider injector imbalance or a bank-specific air distribution problem. Use appropriate test methods available for the vehicle (such as injector balance testing where applicable) and confirm the issue before replacing components.

Professional tip: Treat P0173 as a bank-specific fuel-trim problem and prioritize comparisons between Bank 2 and Bank 1 under the same conditions. If Bank 2 is abnormal while Bank 1 remains controlled, focus on faults that can affect only Bank 2 (localized intake leaks, exhaust leaks on that side, or Bank 2 injector issues) before assuming a system-wide issue.

Possible Fixes

• Repair intake air leaks after the MAF sensor (tighten clamps, repair/replace damaged ducting)
• Repair vacuum/PCV leaks or intake manifold gasket leaks affecting Bank 2
• Repair exhaust leaks upstream of the Bank 2 upstream oxygen sensor
• Clean or correct MAF sensor signal issues and repair related wiring/connectors as needed
• Repair oxygen sensor circuit issues on Bank 2 (wiring, connectors, heater power/ground), and replace the sensor only after confirming it is faulty
• Correct fuel delivery problems (restore proper fuel pressure/volume per specification)
• Address injector imbalance, restriction, or leakage that affects Bank 2
• Correct EVAP purge faults (purge valve not sealing, purge plumbing leaks or routing issues)

Can I Still Drive With P0173?

You can sometimes drive with P0173 for a short time if the engine is running smoothly, but it should be treated as a service-soon condition. Because P0173 indicates abnormal fuel-trim behavior on Bank 2, the air-fuel mixture may be incorrect under certain conditions, which can lead to hesitation, stalling, or reduced power. If the engine is misfiring, running very rough, smells strongly of fuel, or lacks power in a way that affects safe merging or turning, avoid driving and diagnose the problem promptly. Continued operation with an incorrect mixture can increase emissions and may contribute to catalytic converter damage over time.

How Serious Is This Code?

P0173 can range from moderate to serious depending on how far and how often Bank 2 fueling is being corrected. A small but persistent trim issue may mainly cause the MIL and reduced fuel economy, while a larger correction can create drivability problems such as surging, stumbling, or stalling. If the mixture becomes significantly lean or rich, exhaust temperatures and catalyst loading can increase, raising the risk of catalytic converter damage. Because the code indicates the ECM is seeing abnormal trim behavior on Bank 2, it should be diagnosed with scan data and verified tests before extended driving or emissions testing.

Common Misdiagnoses

P0173 is often misdiagnosed as a guaranteed oxygen sensor failure. The code’s official meaning is Fuel Trim Bank 2, which identifies an abnormal fuel-trim condition, not a confirmed bad sensor. Other frequent mistakes include skipping intake leak testing (especially bank-localized leaks), overlooking exhaust leaks upstream of the Bank 2 upstream sensor, and failing to verify fuel pressure under load. Another error is focusing only on a single fuel trim number without comparing Bank 2 to Bank 1 at the same operating condition and without using freeze-frame data to understand when the monitor detected the abnormal trim behavior.

Most Likely Fix

The most likely fix depends on what testing proves is driving the abnormal Bank 2 fuel trim, but repairs often involve restoring correct measured and unmetered airflow (repairing intake/vacuum leaks) or correcting feedback errors (repairing exhaust leaks upstream of the sensor or resolving sensor circuit issues). If live data indicates a true mixture problem, verifying and correcting fuel pressure or Bank 2 injector flow issues becomes the priority. The correct approach is to confirm the Bank 2 trim abnormality with scan data, identify whether it trends lean or rich under specific conditions, then fix the verified root cause and recheck fuel trims after the repair.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.