ASE G1 Auto Maintenance and Light RepairEngine SystemsMedium
A vehicle owner complains that their engine runs rough, hesitates under acceleration, and has poor fuel economy. A diagnostic scan reveals a P0171 trouble code, indicating 'System Too Lean (Bank 1)'. Which of the following is the most likely cause?
- AA clogged exhaust system
- BA restricted fuel return line
- CA faulty oxygen sensor reading rich
- DA leaking intake manifold gasket
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking intake manifold gasket
A P0171 code signifies a lean condition, meaning too much air or not enough fuel. A leaking intake manifold gasket allows unmetered air into the engine, directly causing a lean condition that the oxygen sensor detects.
Why the other options are wrong
- A. A clogged exhaust system would typically cause a lack of power and potentially a rich condition due to exhaust backup, not a lean condition.
- B. A restricted fuel return line would cause excessively high fuel pressure, leading to a rich condition (P0172), not a lean one.
- C. A faulty oxygen sensor reading rich would cause the PCM to lean out the mixture, potentially leading to a P0172 (system too rich) or other codes if it's stuck rich, not P0171.
P0171 (System Too Lean)
A P0171 diagnostic trouble code indicates that the engine's air-fuel mixture is too lean (too much air, not enough fuel) for Bank 1, often caused by vacuum leaks or insufficient fuel delivery.
- Indicates a lean air-fuel mixture.
- Common causes include vacuum leaks (intake gasket).
- Can also be caused by low fuel pressure or dirty MAF sensor.
Memory trick: Lean means air leaks or fuel's weak.