Indicates that front oxygen sensor (at the input of catalytic converter) detects lower than expected percentage of oxygen in exhaust gasses. Could be due to the unbalanced air-to-fuel proportion at the engine, fault oxygen sensor or other systems.


  • Check Engine dashboard light is ON
  • Possible lack of power and car dynamic
  • Possible increase of fuel consumption
  • Possible misfire issues 
  • Unstable work and sound from engine: stumbles, shakes


  • Faulty or dirty mass air fuel (MAF) sensor
  • Dirty or "oiled" air filter
  • Faulty fuel regulators or injectors
  • Vacuum leak
  • Worn-out spark plugs
  • Faulty oxygen sensor
  • Faulty coolant system or thermostat

Severity - moderate

Does not have immediate danger effect to the car or driver, but at longer period could lead to engine overheating and possible damage. Increasing environment pollution.


  • Check and clean MAF and oxygen sensors
  • Inspect vacuum and fuel pipes
  • Inspect and replace, if required, air filter
  • Inspect and replace, if required, fuel pressure regulators or pump
  • Inspect and replace, if required, coolant sensors or thermostat
  • Inspect and replace, if required, spark plugs