Fuel Trim Malfunction (Bank 2)
P0173 indicates the engine computer detected a fuel trim malfunction on Bank 2 (the side of the engine without cylinder #1), meaning the fuel injector pulse width has drifted outside acceptable limits. This occurs when the oxygen sensors detect the air-fuel mixture is either too rich or too lean, and the computer cannot compensate properly.
What You Might Notice
- Check Engine Light (MIL) illumination
- Poor fuel economy
- Black smoke from tailpipe (rich condition)
- Hesitation or misfire at idle or under load
- Rough idle or stalling during startup
Most Common Causes
- 1
Vacuum leak
An unmetered air leak allows extra oxygen into the intake, causing the O2 sensor to detect a lean condition and throwing off fuel trim calculations.
- 2
Faulty oxygen sensor (O2 sensor)
A degraded or contaminated O2 sensor provides incorrect feedback to the engine computer, preventing accurate fuel trim adjustments. Oil contamination in O2 sensor connectors is common.
- 3
Fuel-contaminated or leaking engine oil
Oil degradation or fuel saturation in the crankcase can affect sensor connectors and wiring harness integrity, causing electrical faults in fuel system feedback circuits.
- 4
Turbo air charge hose leak (turbocharged engines)
Cracked or disconnected turbo intercooler hoses allow unmetered boost air into the intake, disrupting air-fuel ratio calculations and fuel trim.
How to Diagnose It
- 1
Check Vacuum leak — An unmetered air leak allows extra oxygen into the intake, causing the O2 sensor to detect a lean condition and throwing off fuel trim calculations.
- 2
Check Faulty oxygen sensor (O2 sensor) — A degraded or contaminated O2 sensor provides incorrect feedback to the engine computer, preventing accurate fuel trim adjustments. Oil contamination in O2 sensor connectors is common.
- 3
Check Fuel-contaminated or leaking engine oil — Oil degradation or fuel saturation in the crankcase can affect sensor connectors and wiring harness integrity, causing electrical faults in fuel system feedback circuits.
- 4
Check Turbo air charge hose leak (turbocharged engines) — Cracked or disconnected turbo intercooler hoses allow unmetered boost air into the intake, disrupting air-fuel ratio calculations and fuel trim.
How to Fix It
- 1
Inspect and repair vacuum leaks
Perform a thorough visual inspection of all vacuum hoses and intake gaskets on Bank 2. Use a smoke test tool to identify leaks, then replace damaged hoses or reseal gaskets as needed.
- 2
Replace oxygen sensor (Bank 2)
Remove and replace the Bank 2 oxygen sensor, cleaning connectors thoroughly to remove any oil contamination. This is the most common fix for this code.
- 3
Check and service fuel system
Perform an oil analysis or change if fuel contamination is suspected. Inspect fuel injectors for leaking and have them cleaned or replaced if they are causing raw fuel to enter the oil.
- 4
Inspect turbo and MAF sensor
On turbocharged engines, check turbo air charge hoses for leaks or disconnection. Inspect and clean the Mass Air Flow (MAF) sensor connector and intake air system for contamination that could affect fuel trim.
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