P0177 — Fuel Composition Sensor Circuit Range/Performance

The fuel-composition sensor circuit has a range or performance fault. The code concerns composition sensing and does not, on its own, identify an incorrect fuel blend.

Affected system: Fuel-composition sensing

Symptoms

• A check-engine warning may appear.
• The engine may hesitate, idle unevenly, start poorly or use more fuel when the sensor reading is unreliable.

Possible causes

• The fuel-composition sensor may operate outside its expected range.
• Corroded contacts or damaged sensor wiring can distort readings; a control-module issue is another possibility.

What to check

• Inspect the fuel-composition sensor wiring and connector for corrosion, loose contacts or damage.
• Compare the sensor live output with vehicle service information and check for applicable controller software updates only after basic circuit checks.

Also written as: P0177-00, P017700, PO177 (some scan tools add the fault-type byte, e.g. -00).