On some vehicles, the OBD II system may monitor the operation of the exhaust gas recirculation system.The EGR system reduces the formation of oxides of nitrogen (NOX) in the exhaust when the engine is under load. OBD II may check for a drop in intake vacuum through the MAP sensor when it commands the EGR valve to open. Some systems also have an EGR valve positon sensor to monitor the opening and closing of the EGR valve. Code for EGR-related faults include P0078 to P0086, and P0400 to P0409. An EGR-related fault code does NOT mean the vehicle is polluting. It might be producing elevated levels of NOX in the exhaust if the EGR system is not operating correctly. The only way to know for sure is to check NOX emissions with an exhaust analyzer. NOX emissions are highest when the engine is under load, which means you may not be able to get accurate readings unless the vehicle is tested in a “loaded mode” by driving it on a dyno, or using a portable exhaust analyzer while test driving the vehicle on the road.

  • P0400       Exhaust Gas Recirculation Flow Malfunction
  • P0401       Exhaust Gas Recirculation Flow Insufficient Detected
  • P0402       Exhaust Gas Recirculation Flow Excessive Detected
  • P0403       Exhaust Gas Recirculation Circuit Malfunction
  • P0404       Exhaust Gas Recirculation Circuit Range/Performance
  • P0405       Exhaust Gas Recirculation Sensor A Circuit Low
  • P0406       Exhaust Gas Recirculation Sensor A Circuit High
  • P0407       Exhaust Gas Recirculation Sensor B Circuit Low
  • P0408       Exhaust Gas Recirculation Sensor B Circuit High
  • P0410       Secondary Air Injection System Malfunction
  • P0411       Secondary Air Injection System Incorrect Flow Detected
  • P0412       Secondary Air Injection System Switching Valve A Circuit Malfunction
  • P0413       Secondary Air Injection System Switching Valve A Circuit Open
  • P0414       Secondary Air Injection System Switching Valve A Circuit Shorted
  • P0415       Secondary Air Injection System Switching Valve B Circuit Malfunction
  • P0416       Secondary Air Injection System Switching Valve B Circuit Open
  • P0417       Secondary Air Injection System Switching Valve B Circuit Shorted
  • P0418       Secondary Air Injection System Relay A circuit Malfunction
  • P0419       Secondary Air Injection System Relay B Circuit Malfunction

Converters typically fail one of two ways: they get too hot, melt and plug up, or the catalyst becomes contaminated and useless. Diagnosing a plugged converter is fairly easy, but diagnosing converter efficiency requires tapping into the OBD II system with a scan tool to check for codes and to compare the upstream and downstream O2 sensor readings.

  • P0420       Catalyst System Efficiency Below Threshold (Bank 1)
  • P0421       Warm Up Catalyst Efficiency Below Threshold (Bank 1)
  • P0422       Main Catalyst Efficiency Below Threshold (Bank 1)
  • P0423       Heated Catalyst Efficiency Below Threshold (Bank l)
  • P0424       Heated Catalyst Temperature Below Threshold (Bank 1)
  • P0430       Catalyst System Efficiency Below Threshold (Bank 2)
  • P0431       Warm Up Catalyst Efficiency Below Threshold (Bank 2)
  • P0432       Main Catalyst Efficiency Below Threshold (Bank 2)
  • P0433       Heated Catalyst Efficiency Below Threshold (Bank 2)
  • P0434       Heated Catalyst Temperature Below Threshold (Bank 2)
  • P0440       Evaporative Emission Control System Malfunction
  • P0441       Evaporative Emission Control System Incorrect Purge flow
  • P0442       Evaporative Emission Control System leak Detected (small leak)
  • P0443       Evaporative Emission Control System Purge Control Valve circuit Malfunction
  • P0444       Evaporative Emission Control System Purge Control Valve Circuit Open
  • P0445       Evaporative Emission Control System Purge Control Valve Circuit Shorted
  • P0446       Evaporative Emission Control System Vent Control Circuit Malfunction
  • P0447       Evaporative Emission Control System Vent Control Circuit Open
  • P0448       Evaporative Emission Control System Vent Control Circuit Shorted
  • P0449       Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction
  • P0450       Evaporative Emission Control System Pressure Sensor Malfunction
  • P0451       Evaporative Emission Control System Pressure Sensor Range/Performance
  • P0452       Evaporative Emission Control System Pressure Sensor Low Input
  • P0453       Evaporative Emission Control System Pressure Sensor High Input
  • P0454       Evaporative Emission Control System Pressure Sensor Intermittent
  • P0455       Evaporative Emission Control System Tank Detected (gross leak)
  • P0460       Fuel Level Sensor Circuit Malfunction
  • P0461       Fuel Level Sensor Circuit Range/Performance
  • P0462       Fuel level Sensor Circuit Low Input
  • P0463       Fuel level Sensor Circuit High Input
  • P0464       Fuel level Sensor Circuit Intermittent
  • P0465       Purge Flow Sensor Circuit Malfunction
  • P0466       Purge flow Sensor Circuit Range/Performance
  • P0467       Purge Flow Sensor Circuit Low Input
  • P0468       Purge flow Sensor Circuit High Input
  • P0469       Purge flow Sensor Circuit Intermittent
  • P0470       Exhaust Pressure Sensor Malfunction
  • P0471       Exhaust Pressure Sensor Range/Performance
  • P0472       Exhaust Pressure Sensor Low
  • P0473       Exhaust Pressure Sensor High
  • P0474       Exhaust Pressure Sensor Intermittent
  • P0475       Exhaust Pressure Control Valve Malfunction
  • P0476       Exhaust Pressure Control Valve Range/Performance
  • P0477       Exhaust Pressure Control Valve Low
  • P0478       Exhaust Pressure Control Valve High
  • P0479       Exhaust Pressure Control Valve Intermittent
  • P0480       Cooling Fan 1 Control Circuit Malfunction
  • P0481       Cooling Fan 2 Control Circuit Malfunction
  • P0482       Cooling Fan 3 Control Circuit Malfunction
  • P0483       Cooling Fan Rationality Check Malfunction
  • P0484       Cooling Fan Circuit Over Current
  • P0485       Cooling Fan Power/Ground Circuit Malfunction