What causes fault code P0173?

• Most likely: Vacuum leak specifically affecting Bank 2 intake • Likely: Faulty or contaminated Mass Air Flow (MAF) sensor • Possible: Faulty Oxygen Sensor (O2) on Bank 2 (sensor bias) • Rare: Defective fuel pressure regulator or leaking fuel injectors on Bank 2

How do you fix fault code P0173?

STEP 1: Inspect all vacuum hoses and intake boots on Bank 2 for cracks or leaks. Ensure the oil dipstick and oil cap are properly sealed. STEP 2: Clean the MAF sensor using specialized cleaner. Monitor MAF readings; idle values should typically be 2-5 g/s depending on engine displacement. STEP 3: Monitor Oxygen Sensor (Bank 2, Sensor 1) voltage switching. It should oscillate rapidly between 0.1V and 0.9V. If stuck, replace the sensor. STEP 4: Verify fuel pressure is within OEM specifications and check for leaking injectors. Reset fuel trim adaptations after repair.

Powertrain (Engine & Gearbox)Standard EOBD

P0173 - Fuel Trim Malfunction (Bank 2)

Q: What are the symptoms of fault code P0173?

• Check Engine Light (MIL) illumination • Rough idle • Increased fuel consumption • Hesitation during acceleration and loss of engine power • Failed emission test

Last updated:1 February 2026

Related:Mass Air Flow Sensor (MAF), Oxygen Sensor (O2), Insprutare, Fuel Pressure Regulator, Vakuumslangar

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0100

Mass or Volume Air Flow A Circuit Malfunction

P0101

Mass or Volume Air Flow Circuit Range/Performance Problem

P0102

Mass Air Flow (MAF) Circuit Low Input

P0103

Mass or Volume Air Flow Circuit High Input

Technical Description

P0173 indicates that the Engine Control Unit (ECU) has reached its calibrated limit for fuel trim adjustment on Bank 2. When the Long Term Fuel Trim (LTFT) exceeds its correction threshold (typically +/- 20-25%), the ECU can no longer maintain a stoichiometric air-fuel ratio. Unlike P0171/P0174 which specify lean conditions, P0173 is a general malfunction indicating the system is unable to stabilize the mixture, often caused by sensor bias or unmetered air/fuel delivery issues.

Possible Causes

Vacuum leak specifically affecting Bank 2 intake

Faulty or contaminated Mass Air Flow (MAF) sensor

Faulty Oxygen Sensor (O2) on Bank 2 (sensor bias)

Defective fuel pressure regulator or leaking fuel injectors on Bank 2

Symptoms

Check Engine Light (MIL) illumination

Rough idle

Increased fuel consumption

Hesitation during acceleration and loss of engine power

Failed emission test

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P0173 - Fuel Trim Malfunction (Bank 2)

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P0173 - Fuel Trim Malfunction (Bank 2)

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