P0170
Fuel Trim Malfunction (Bank 1)
The ECU detects that short-term and long-term fuel trim values on bank 1 are outside acceptable limits. Engine cannot maintain stoichiometric mixture — typically a vacuum leak, fuel pressure issue, or failing MAF/O2 sensor.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Vacuum leak (intake gasket, hose, PCV)Common
- Fuel pressure too low or too highPossible
- MAF sensor reading driftedPossible
Parts involved
- Intake System
- Fuel Pressure Regulator
- Mass Air Flow Sensor
- Oxygen Sensor B1s1
P0170 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.