P0173
Fuel Trim Bank 2.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 73
- FaultThe specific fault in that area
What it means
P0173 is the generic code for a fuel trim fault on bank 2, where the bank 2 oxygen sensor keeps reporting a rich or lean mixture and the control unit has run out of fuel trim adjustment. The SAE definition is 'Fuel Trim Malfunction (Bank 2)', and in practice it is most often set when the system is trimming too far in the lean direction (too little fuel or too much unmetered air). The code only applies to engines with two cylinder banks, and bank 2 is the side that does not contain cylinder number 1.
Symptoms
- Engine warning light on
- Rough or unstable idle
- Hesitation or stumbling under acceleration
- Poor fuel economy
- Occasional misfire or surging at cruise
- Hard starting or stalling in some cases
Possible causes
- Vacuum leak or unmetered air entering on bank 2, such as a split intake hose, a leaking intake manifold gasket or a perished vacuum linelikely
- Faulty or contaminated mass air flow sensor under-reading the air entering the enginelikely
- Restricted or weak fuel delivery, such as a clogged fuel filter (where fitted), weak fuel pump or faulty pressure regulatorpossible
- Clogged or leaking fuel injector(s) on bank 2possible
- Exhaust leak upstream of the bank 2 oxygen sensor letting in outside air and giving a false lean readingpossible
- Faulty or slow bank 2 upstream oxygen or air/fuel ratio sensor, or damaged wiring to itpossible
- Crankcase ventilation fault or leaking brake servo hose allowing extra air inrare
How to diagnose
- Read the stored codes and freeze frame data, noting engine load, temperature, speed and fuel trim values when the fault set, and check for related codes such as P0171, misfire or MAF codes.
- Inspect the intake pipework, air filter housing, vacuum hoses and PCV hoses for splits, loose clips and disconnected joints, particularly around bank 2.
- Compare short and long term fuel trims on both banks at idle and at about 2500 rpm; if both banks are lean suspect air metering or fuel supply, if only bank 2 is lean suspect a local leak or injector problem.
- Check live mass air flow readings against expected values for the engine at idle and compare with the manufacturer data, after inspecting the sensor and its connector for contamination or damage.
- Smoke test the intake and check for exhaust leaks ahead of the bank 2 oxygen sensor.
- Measure fuel pressure and check for pressure drop after shutdown, using the manufacturer's specification for the engine.
- Check bank 2 oxygen or air/fuel ratio sensor wiring, connector and live response, and test the injectors on that bank for correct operation and spray pattern.
- Clear the code after repair and road test, then recheck fuel trims to confirm they are back within normal range.
How to fix
- Repair or replace leaking intake hoses, gaskets or vacuum lines found during the leak test.
- Clean or replace the mass air flow sensor if it is confirmed to be reading incorrectly.
- Repair the exhaust leak upstream of the bank 2 sensor.
- Restore fuel supply by replacing a blocked filter, a weak pump or a faulty pressure regulator as diagnosed.
- Clean or replace faulty injectors on bank 2.
- Replace the bank 2 oxygen or air/fuel ratio sensor, or repair its wiring, only if testing shows it is at fault.
Can I keep driving?
The car can usually be driven for a short time, but a lean mixture can cause misfires, overheating of the combustion chambers and damage to the catalytic converter, so have it checked soon. Avoid heavy loads and hard acceleration, and stop driving if the engine runs very roughly or the warning light flashes.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.