P0173 — Fuel Trim Malfunction (Bank 2)

P0173 — Fuel Trim Malfunction (Bank 2). A fault in the fuel delivery and mixture control. Short, gentle driving to a shop is usually possible. Typical causes: vacuum or intake leaks skewing the mixture, faulty fuel rail pressure sensor or wiring, faulty fuel pressure regulator.

powertrain (engine/transmission)

Is it safe to drive?

Diagnose P0173 promptly If the engine runs steadily, P0173 usually justifies a cautious trip to a repair shop rather than immediate shutdown. Avoid extended driving if the vehicle stalls, misfires, loses substantial power, or smells strongly of fuel, because a persistent fuel-trim error can increase emissions and eventually damage the catalytic converter.

Typical symptoms

The check-engine light or emission-monitoring warning may illuminate with P0173 stored in the powertrain control module.
A rough idle, hesitation, or stumble may occur when the controller is making a large Bank 2 fuel-trim correction.
Engine misfires or loss of power can develop if the mixture becomes sufficiently lean or rich.
Fuel consumption may increase because the controller is adding extra fuel to correct the commanded mixture.
P0173 may be the only noticeable symptom, especially if the learned correction remains only slightly outside its permitted range.
Live scan data may show Bank 2 short-term or long-term trims at the maximum correction limit while commanded air-fuel values remain normal.

Common causes

An unmetered-air or vacuum leak in the Bank 2 intake tract, throttle-body gasket, intake manifold, or connected hose can trigger P0173.
A contaminated or inaccurate mass-air-flow sensor may report too much or too little air, forcing the engine controller to make large Bank 2 fuel-trim corrections.
Low fuel pressure, a restricted fuel filter, or an inadequate pressure regulator can prevent Bank 2 injectors from delivering the required fuel.
Dirty, leaking, or flow-imbalanced Bank 2 injectors can create a mixture difference that eventually sets P0173.
A faulty Bank 2 oxygen sensor, especially the upstream sensor used for fuel-trim feedback, or damaged sensor wiring can cause an incorrect mixture correction and P0173.
An exhaust leak ahead of the Bank 2 oxygen sensor can expose the sensor to false air and contribute to an incorrect fuel-trim command.
A failed positive crankcase ventilation or exhaust-gas-recirculation valve can admit unmetered air and drive the fuel trims outside their learned limits.

How to diagnose with an OBD scanner

Verify P0173 with live scan data Connect an OBD2 scanner such as ELM327 with AIScanAuto, read stored and pending codes, and save freeze-frame data before clearing anything. Compare Bank 2 short-term and long-term fuel trims with the commanded air-fuel values, while also checking intake-air temperature, mass-air-flow, manifold-pressure, and oxygen-sensor data. P0173 confirms a correction problem, not the failed component by itself.
Inspect the Bank 2 air-intake system Check the mass-air-flow sensor element, throttle-body gasket, intake-manifold seams, brake-booster hose, positive crankcase ventilation lines, and other vacuum hoses for looseness, contamination, or cracking. A low-pressure smoke test can reveal an unmetered-air leak, but do not exceed the test equipment's pressure rating.
Test fuel pressure and injector delivery Allow the engine to cool and relieve fuel pressure according to the vehicle procedure before opening the fuel system. Measure pressure against the manufacturer specification, inspect the filter and regulator, and then perform an injector balance or flow test on Bank 2. A restricted feed or unequal injector delivery can cause P0173 even when the engine idles normally.
Validate the oxygen sensor and related wiring Inspect the Bank 2 oxygen-sensor connectors and wiring for corrosion, heat damage, and resistance problems. Review sensor response in live data and confirm that the upstream sensor is switching appropriately. Replace the sensor only if its performance or electrical testing fails; otherwise, an exhaust leak or fuel-supply fault may still cause P0173.
Repair, clear, and confirm the correction Correct the confirmed leak, pressure fault, injector issue, or sensor fault. Clear the powertrain codes only after recording the original data, then complete a road test and recheck Bank 2 fuel trims. P0173 should stay inactive and the trims should move closer to their normal target range; repeated codes require further diagnosis rather than repeated clearing.

Approximate repair cost

A typical independent-shop estimate for diagnosing and repairing P0173 is approximately USD $250-$1,500, depending on whether the fault is a hose leak, sensor, fuel-pressure problem, or injector repair. These figures are estimates and vary by region.

Common questions

Does P0173 always mean Bank 2 is running lean?

No. P0173 identifies an abnormal Bank 2 fuel-trim correction, and lean operation is a common possibility, but the code alone cannot prove the mixture direction or identify the failed part.

Is it safe to drive with P0173 stored?

A stable vehicle can usually be driven cautiously to a repair shop, but prolonged operation with a large correction can damage the catalytic converter. Stop driving if P0173 is accompanied by severe misfire, stalling, major power loss, or a strong fuel odor.

Does P0173 always indicate a defective injector?

No. An injector imbalance is only one cause. P0173 may also result from unmetered intake air, low fuel pressure, an oxygen-sensor fault, an exhaust leak, or a contaminated air-flow sensor.

What should I examine first when checking P0173?

Start with stored freeze-frame data and live Bank 2 fuel-trim values, then inspect the intake and vacuum hoses before testing fuel pressure, injectors, and oxygen-sensor operation. This sequence avoids replacing a sensor when a simple leak is causing P0173.

When your vehicle's engine control module detects that the air-fuel mixture on one side of the engine is running too rich or too lean for an extended period, it logs trouble code P0173. This code specifically refers to a fuel trim malfunction on Bank 2, which is the engine bank that does not contain cylinder number one. The fuel trim system is the PCM's way of adjusting fuel delivery in real time — adding more fuel when the mixture runs lean or cutting fuel when it runs rich — to maintain the ideal 14.7:1 air-fuel ratio under all conditions. P0173 triggers only after the control module observes this trim correction out of specification repeatedly across multiple drive cycles, which is why a single brief anomaly will not set the light.

The oxygen sensors mounted in the exhaust stream feed constant data back to the engine computer, which uses that information to fine-tune injector pulse width. When Bank 2's fuel trim drifts outside acceptable limits, the PCM tries to compensate but eventually gives up and illuminates the Check Engine light. Freeze frame data stored alongside P0173 is valuable because it records the exact engine load, RPM, coolant temperature, and vehicle speed present when the fault first appeared. Reproducing those conditions during diagnosis makes it far easier to trigger the problem consistently.

Symptoms of P0173

Persistent fuel odor, especially when the mixture runs rich
Noticeable loss of power when accelerating or climbing grades
Rough idle, stumbling, or hesitation during throttle tip-in
Check Engine light remains illuminated on the dashboard
Measurably higher fuel consumption compared to normal
In rare cases, intermittent stalling under load

Common Causes of P0173

Fuel pump output dropping below specification or fuel filter restriction
Fuel injectors clogged with debris or leaking externally
Faulty fuel pressure regulator allowing unregulated rail pressure
Fuel contamination or wrong octane fuel disrupting combustion
Malfunctioning fuel rail pressure sensor or damaged wiring to that sensor
Vacuum leaks on the intake side drawing unmetered air into Bank 2's cylinders

How to Diagnose P0173

Plug in a quality OBD2 scanner, retrieve P0173 and any related codes, then record the freeze frame data — these parameters show exactly what the engine was doing when the fault triggered.
Review all stored codes together; codes such as P0100 through P0105 (MAF and MAP sensor faults) frequently accompany fuel trim issues and point toward the actual root cause.
Perform a visual inspection of the Bank 2 fuel and intake wiring harnesses, checking for cracked insulation, corroded terminals, loose connectors, or chafed wires near heat sources or moving components.
Measure fuel system pressure at the fuel rail with a gauge while the engine idles and again under manifold vacuum to compare against manufacturer specifications for both conditions.
Monitor short-term and long-term fuel trims in live data mode; strongly positive trim values indicate a lean condition requiring more fuel, while strongly negative values show a rich condition.
Clear all codes, complete a thorough drive cycle that reproduces the freeze frame conditions (load, RPM, temperature, speed), then rescan to confirm whether P0173 returns.

Repair Options and Costs

The appropriate repair depends entirely on which component is actually responsible for the trim malfunction:

Fuel filter replacement: $80–$200
Fuel pressure regulator or fuel rail pressure sensor replacement: $150–$450
Injector cleaning or replacement: $200–$800
Fuel pump replacement: $400–$900

Addressing P0173 typically costs between $80 and $900 total when parts and labor are combined at independent shop rates. The least expensive scenario involves a simple fuel filter swap, while the most costly involves a complete fuel pump assembly replacement. Diagnosing correctly before purchasing parts is the single most effective way to keep total repair costs reasonable — replacing the fuel pump when the actual culprit is a vacuum leak wastes both money and time.

Can You Drive with P0173?

Caution is the right approach here. While the engine may run without obvious symptoms in mild cases, a persistent fuel trim malfunction can produce hesitation that becomes dangerous in traffic situations, and a severely rich mixture risks coating spark plugs and potentially damaging catalytic converters over time. Short trips to a trusted repair facility are acceptable, but extended highway driving or heavy traffic situations should be avoided until the underlying cause is identified and corrected.