◆OBD Codes

P0178

Fuel Composition Sensor Circuit Low Input

Moderatesensor

Overview

The P0178 diagnostic trouble code means the PCM has identified a fuel composition sensor circuit low input problem. This code indicates that a sensor or its circuit is not providing expected readings to the engine management system. Faulty sensor data can cause the PCM to make incorrect adjustments, affecting engine performance.

Symptoms

  • Check Engine Light (MIL) illuminated
  • Erratic engine behavior
  • Engine running rough
  • Intermittent stalling

Common Causes

  • Sensor contamination or buildup
  • Physical damage to sensor
  • Incorrect sensor installed
  • Wiring issues (open, short, or high resistance)

Diagnosis Steps

  1. 1Connect an OBD-II scanner and read all stored trouble codes and freeze frame data
  2. 2Locate the specific sensor referenced by the code
  3. 3Inspect the sensor connector for corrosion, damage, or loose pins
  4. 4Test the sensor output using a multimeter or scan tool live data
  5. 5Check the wiring between the sensor and the PCM for opens or shorts

Estimated Repair Cost

$75 — $350

Cost varies by vehicle, location, and repair shop. Includes parts and labor.

Commonly Affected Vehicles

Subaru ImprezaKia ForteHonda CivicVolkswagen Golf

Related Codes

P0120P0335P0115

More Sensor Codes

P0070Ambient Air Temperature Sensor Circuit
Low
P0071Ambient Air Temperature Sensor Range/Performance
Low
P0072Ambient Air Temperature Sensor Circuit Low
Low
P0073Ambient Air Temperature Sensor Circuit High
Low
P0074Ambient Air Temperature Sensor Circuit Intermittent
Low
P0095Intake Air Temperature Sensor 2 Circuit
Low

Frequently Asked Questions

Most automotive sensors last 80,000-150,000 miles, though this varies by sensor type and driving conditions. Oxygen sensors and temperature sensors tend to degrade gradually over time.