P0172 - System Too Rich (Bank 1)
System
Powertrain (Engine & Gearbox)
Technical Description
The ECM has reached its maximum fuel subtraction threshold (Long Term Fuel Trim below -20% to -25%) to compensate for excessive fuel delivery or restricted airflow on Bank 1. This causes spark plug fouling and cylinder wall oil dilution.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Short Term Fuel Trim (STFT Bank 1) | Voltage (%) | -20.0% to -5.0% | Idle and part load |
| Long Term Fuel Trim (LTFT Bank 1) | Voltage (%) | < -20.0% (fault condition) | Under cruise |
| Fuel Pressure Regulator (Vacuum referenced) | Pressure (bar) | 3.0 bar (idle with vacuum) / 3.5 bar (vacuum line disconnected) | Fuel pressure gauge |
Idle and part load
Under cruise
Fuel pressure gauge
Oscilloscope Waveform Analysis
1 s/div, 200 mV/divPossible Causes
Leaking, dripping fuel injector
Failed fuel pressure regulator diaphragm leaking fuel directly into intake vacuum line
Contaminated MAF sensor over-reporting airflow
Severely restricted air filter or stuck-open EVAP purge valve
Symptoms
4
Recommended Solution
Pull vacuum line off fuel pressure regulator; if raw fuel is present, regulator diaphragm is ruptured.
Perform fuel pressure drop test with key off to check for leaking injectors.
Check engine oil dipstick for fuel odor and oil dilution.
Mechanic's Tip
Pull the dipstick and smell the oil! Fuel-diluted engine oil continually boils off fuel vapor into the PCV intake stream, causing rich codes. Always change engine oil and filter after fixing the root cause!