What does this code mean?
P0171/P0172 refer to bank 1; P0174/P0175 to bank 2. Lean or rich describes the controller's mixture assessment, not a direct measurement of a failed part. Compare fuel trims at idle and under load, inspect air and exhaust leaks, and test fuel supply or sensor readings using vehicle-specific limits.
Sources and interpretation [1]
A code identifies a detected condition, not a part that must automatically be replaced.
Manufacturer communications
5Original wording in English
FORD, MERCURY
Reported code forms P0172
MALFUNCTION INDICATOR LAMP COMING ON WITH DIAGNOSTIC TROUBLE CODES (DTCS) P0171, P0172, P0174 AND/OR P0175 (FUEL SYSTEM TOO RICH OR LEAN). *TT
FORD, MERCURY
Reported code forms P0172
MALFUNCTION INDICATOR LAMP (MIL) ON WITH CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODES (DTC'S) P0172/P0175 (SYSTEM TOO RICH). *TT
FORD, MERCURY
Reported code forms P0172
SOME VEHICLES MAY EXHIBIT FUEL DIAGNOSTIC TROUBLE CODES P0171, P0172, P0174, P0175 OR MASS AIR FLOW DIAGNOSTIC TROUBLE CODES P0102, P0103, P0112, P0113. *TT
FORD, MERCURY
Reported code forms P0172
SOME VEHICLES EQUIPPED WITH A 3.0L ENGINE MAY EXHIBIT A MALFUNCTION INDICATOR L AMP ILLUMINATION AND DIAGNOSTIC TROUBLE CODES P0172/P0175, HARSH TRANSMISSION ENGAGEMENTS WHEN COLD ONLY OR A SPARK KNOCK ON ACCELERATION…
FORD, MERCURY
Reported code forms P0172
SOME VEHICLES EQUIPPED WITH A 3.0L FFV ENGINE MAY EXHIBIT MALFUNCTION INDICATOR LAMP ILLUMINATION AND DIAGNOSTIC TROUBLE CODES P0172/P0175. *TT
Check vehicle applicability
3A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Sources and interpretation
These explanations are editorial summaries of the linked manufacturer material. PDF page numbers count from the first file page. Original publications are in English; translations are reference aids. A cited edition is not a claim that it is the manufacturer's latest instruction.
- 2024–2025 MY OBD System Operation Summary for Gasoline Engines ↗
Ford Motor Company · PDF page(s) 21, 22 ·
Source check date · 2026-10-27
Can I keep driving?
A code alone cannot establish whether driving is safe. A flashing malfunction light, severe shaking, overheating, oil-pressure warning or loss of braking or steering needs prompt attention. Stop in a safe place and seek assistance when control or engine safety is affected. Follow the vehicle handbook.