P0173: Fuel Trim Malfunction Bank 2

What it means

Fuel trims on bank 2 exceed adaptation limits in either lean or rich direction. The ECU cannot maintain stoichiometric mixture within calibrated boundaries.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The ECU adjusts fuel delivery using feedback from bank 2’s oxygen sensor to keep that bank’s mixture near its target. P0173 sets when the learned fuel corrections for bank 2 reach their allowed limit in either the lean or rich direction.

Common causes

Most likely first.

1. Oxygen sensor bank 2 aged
2. Vacuum leak bank 2 side

Symptoms

• Check engine light on
• Rough or unstable idle
• Hesitation or reduced power
• Poor fuel economy

How to diagnose it

1. Read freeze-frame data and compare short- and long-term fuel trims for bank 2 with bank 1 to see whether the correction is bank-specific and when it occurs.
2. Inspect the intake hoses, vacuum lines, and gaskets serving bank 2 for leaks that could admit unmetered air.
3. Check bank 2’s oxygen-sensor wiring and connector for damage, contamination, or loose pins.
4. Use live data to see whether bank 2’s oxygen-sensor response agrees with the fuel-trim direction as the engine warms and runs.
5. If the trims remain abnormal, test for exhaust leaks near the bank 2 sensor and verify fuel delivery and sensor operation to the manufacturer's specifications.

Common fixes

• Repair a vacuum hose or intake leak affecting bank 2.
• Repair damaged bank 2 oxygen-sensor wiring or connector contacts.
• Repair an exhaust leak near the bank 2 oxygen sensor.
• Replace the bank 2 oxygen sensor if testing confirms it is aged or unresponsive.

Can I keep driving?

If the engine runs smoothly, a short, gentle drive may be possible, but avoid hard acceleration and monitor for worsening roughness or power loss. Continued incorrect fueling can increase fuel use and emissions and may damage the catalytic converter.