P0173 OBD2 Code Guide
P0173 is a fuel trim diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
Fuel Trim Malfunction (Bank 2)
Code P0173 indicates a general fuel trim malfunction on Bank 2. The Powertrain Control Module (PCM) has detected that it cannot maintain the proper 14.7:1 air-fuel ratio by adjusting the short-term and long-term fuel trims within its programmed parameters for Bank 2. This suggests a consistent rich or lean condition specific to the engine bank that does not contain cylinder #1, leading to inefficient combustion, increased emissions, and potential long-term damage to the catalytic converter.
Code verification standard
P0173 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0173 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0173 diagnostic path
- 11. Inspect for Vacuum Leaks on Bank 2: Carefully check all vacuum lines, intake manifold gaskets, PCV valve, and brake booster connections specific to Bank 2. Use a smoke machine or an unlit propane torch/carb cleaner spray to pinpoint any leaks that could introduce unmetered air.
- 22. Monitor Oxygen Sensor Readings (Bank 2): Using a scan tool, observe live data for the upstream (Sensor 1) and downstream (Sensor 2) oxygen sensors on Bank 2. Look for slow response times, stuck rich/lean readings, or values that do not fluctuate as expected. Also, inspect wiring and connectors for damage or corrosion.
- 33. Evaluate Fuel Delivery Components for Bank 2: Check fuel pressure at the rail to ensure it's within specifications. While a low fuel pressure affects both banks, a clogged or faulty fuel injector on Bank 2 could cause a localized issue. Perform an injector balance test if available.
- 44. Inspect Mass Air Flow (MAF) Sensor and Air Intake System: Although P0173 is bank-specific, a dirty or failing MAF sensor can cause widespread fuel trim issues. Clean the MAF sensor with appropriate cleaner and inspect the entire air intake system for leaks or obstructions *before* the MAF sensor. Ensure the air filter is clean and properly seated.
- 55. Check for Exhaust Leaks on Bank 2: Inspect the exhaust system for any leaks *before* the upstream oxygen sensor on Bank 2. An exhaust leak in this area can draw in ambient air, causing the O2 sensor to falsely report a lean condition and skew fuel trim calculations.
Verify before repair
- - Repair vacuum leaks on Bank 2 (e.g., replace intake manifold gaskets, PCV valve, or damaged vacuum hoses).
- - Replace faulty upstream or downstream oxygen sensors on Bank 2.
- - Clean or replace the Mass Air Flow (MAF) sensor if it's contaminated or malfunctioning.
- - Address fuel delivery issues, such as replacing a clogged fuel filter, a weak fuel pump, or a sticking/clogged fuel injector on Bank 2.
Vehicle-specific pages to check next
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