Not all OBDII codes cause an engine light, OBD codes or DTC (diagnostic trouble code) are stored by the ECU (Engine Control Unit) or PCM (Powertrain Control Module) to point the mechanic to the cause.  The engine light is an indicator of problems.  The engine light on its own does not provide details, but reading the stored DTC is a good pointer to where the issue is.

If a DTC is stored in your ECU, it doesn’t necessary mean that there will be an engine light.  In fact no other indicator may be present.  The engine light is typically reserved for drivability or emission problems.  If you’re a DIY person, you should invest in a good OBDII scanner, now available with bluetooth.

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OBD / OBDII DTC stored in the ECU (May be causing an engine light):

1. Powertrain Codes (P0XXX, P2XXX, P34XX – P39XX)
   • P0001 – P0099: Fuel and Air Metering
   • P0100 – P0199: Fuel and Air Metering
   • P0200 – P0299: Fuel and Air Metering (Injector Circuit)
   • P0300 – P0399: Ignition System or Misfire
   • P0400 – P0499: Auxiliary Emissions Controls
   • P0500 – P0599: Vehicle Speed Controls and Idle Control System
   • P0600 – P0699: Computer Output Circuit
   • P0700 – P0799: Transmission
   • P0800 – P0899: Transmission
   • P0900 – P0999: clutch and auxiliary emission control
   • P1XXX: Manufacturer-specific codes

Powertrain Codes P0400 – P0499

• P0400 – Exhaust Gas Recirculation Flow Malfunction
  • causes: Faulty EGR valve, blocked EGR, vacuum leaks, EGR solenoid.
• P0401 – Exhaust Gas Recirculation Flow Insufficient Detected
  • causes: Clogged or partially blocked EGR, EGR valve, EGR solenoid, or sensor.
• P0402 – Exhaust Gas Recirculation Flow Excessive Detected
  • causes: Faulty EGR valve stuck open, EGR solenoid, sensor.
• P0403 – Exhaust Gas Recirculation Circuit Malfunction
  • causes: Faulty EGR solenoid, wiring or connector, ECM.
• P0404 – Exhaust Gas Recirculation Circuit Range/Performance
  • causes: Faulty EGR valve, sensor, carbon buildup in EGR.
• P0405 – Exhaust Gas Recirculation Sensor ‘A’ Circuit Low
  • causes: Short to ground in the sensor circuit, EGR sensor, wiring.
• P0406 – Exhaust Gas Recirculation Sensor ‘A’ Circuit High
  • causes: Short to power in the sensor circuit, EGR sensor, wiring.
• P0407 – Exhaust Gas Recirculation Sensor ‘B’ Circuit Low
  • causes: Same as P0405, but sensor ‘B’.
• P0408 – Exhaust Gas Recirculation Sensor ‘B’ Circuit High
  • causes: Same as P0406, but sensor ‘B’.
• P0409 – Exhaust Gas Recirculation Sensor ‘A’ Circuit
  • causes: Faulty EGR sensor, wiring or connector, carbon buildup.

OBD / OBDII DTC P0400 – P0499

• P0410 – Secondary Air Injection System Malfunction
  • causes: Faulty air injection pump, blocked hoses, faulty one-way check valve, relay or solenoid.
• P0411 – Secondary Air Injection System Incorrect Flow Detected
  • causes: Blocked hoses or tubes, faulty air injection pump, check valve.
• P0412 – Secondary Air Injection System Switching Valve ‘A’ Circuit Malfunction
  • causes: Faulty air injection system switching valve, wiring or connector.
• P0413 – Secondary Air Injection System Switching Valve ‘A’ Circuit Open
  • causes: Wiring or connector, faulty switching valve.
• P0414 – Secondary Air Injection System Switching Valve ‘A’ Circuit Shorted
  • causes: Short circuit in wiring or connectors, faulty switching valve.
• P0415 – Secondary Air Injection System Switching Valve ‘B’ Circuit Malfunction
  • causes: Same as P0412, but valve ‘B’.
• P0416 – Secondary Air Injection System Switching Valve ‘B’ Circuit Open
  • causes: Same as P0413, but valve ‘B’.
• P0417 – Secondary Air Injection System Switching Valve ‘B’ Circuit Shorted
  • causes: Same as P0414, but valve ‘B’.
• P0418 – Secondary Air Injection System Relay ‘A’ Circuit Malfunction
  • causes: Faulty relay, wiring or connector, control circuit.
• P0419 – Secondary Air Injection System Relay ‘B’ Circuit Malfunction
  • causes: Same as P0418, but relay ‘B’.

OBD / OBDII DTC P0400 – P0499

• P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
  • causes: Faulty catalytic converter, oxygen sensor, exhaust leaks, fuel mixture.
• P0421 – Warm Up Catalyst Efficiency Below Threshold (Bank 1)
  • causes: Faulty warm-up catalytic converter, oxygen sensor, engine misfires, exhaust leaks.
• P0422 – Main Catalyst Efficiency Below Threshold (Bank 1)
  • causes: Same as P0420.
• P0423 – Heated Catalyst Efficiency Below Threshold (Bank 1)
  • causes: Faulty heated catalytic converter, oxygen sensor, engine performance.
• P0424 – Heated Catalyst Temperature Below Threshold (Bank 1)
  • causes: Faulty heated catalytic converter, temperature sensor, exhaust system.
• P0425 – Catalyst Temperature Sensor Circuit (Bank 1)
  • causes: Faulty catalyst temperature sensor, wiring or connector.
• P0426 – Catalyst Temperature Sensor Range/Performance (Bank 1)
  • causes: Faulty sensor, wiring, catalytic converter.
• P0427 – Catalyst Temperature Sensor Circuit Low Input (Bank 1)
  • causes: Short to ground, sensor, wiring.
• P0428 – Catalyst Temperature Sensor Circuit High Input (Bank 1)
  • causes: Short to power, sensor, wiring.
• P0429 – Catalyst Heater Control Circuit (Bank 1)
  • causes: Faulty catalyst heater, wiring or connector, control circuit.

OBD / OBDII DTC P0400 – P0499

• P0430 – Catalyst System Efficiency Below Threshold (Bank 2)
  • causes: Same as P0420, but bank 2.
• P0431 – Warm Up Catalyst Efficiency Below Threshold (Bank 2)
  • causes: Same as P0421, but bank 2.
• P0432 – Main Catalyst Efficiency Below Threshold (Bank 2)
  • causes: Same as P0422, but bank 2.
• P0433 – Heated Catalyst Efficiency Below Threshold (Bank 2)
  • causes: Same as P0423, but bank 2.
• P0434 – Heated Catalyst Temperature Below Threshold (Bank 2)
  • causes: Same as P0424, but bank 2.
• P0435 – Catalyst Temperature Sensor Circuit (Bank 2)
  • causes: Same as P0425, but bank 2.
• P0436 – Catalyst Temperature Sensor Range/Performance (Bank 2)
  • causes: Same as P0426, but bank 2.
• P0437 – Catalyst Temperature Sensor Circuit Low Input (Bank 2)
  • causes: Same as P0427, but bank 2.
• P0438 – Catalyst Temperature Sensor Circuit High Input (Bank 2)
  • causes: Same as P0428, but bank 2.
• P0439 – Catalyst Heater Control Circuit (Bank 2)
  • causes: Same as P0429, but bank 2.

OBD / OBDII DTC P0400 – P0499

• P0440 – Evaporative Emission Control System Malfunction
  • causes: Faulty or loose fuel cap, purge or vent solenoid, leak in the EVAP system, charcoal canister.
• P0441 – Evaporative Emission Control System Incorrect Purge Flow
  • causes: Faulty purge solenoid, EVAP system leak, blocked or cracked hoses.
• P0442 – Evaporative Emission Control System Leak Detected (Small Leak)
  • causes: Loose or faulty fuel cap, small holes or cracks in EVAP hoses, faulty vent or purge solenoids.
• P0443 – Evaporative Emission Control System Purge Control Valve Circuit Malfunction
  • causes: Faulty purge control valve, wiring or connector.
• P0444 – Evaporative Emission Control System Purge Control Valve Circuit Open
  • causes: Faulty purge control valve, wiring or connector.
• P0445 – Evaporative Emission Control System Purge Control Valve Circuit Shorted
  • causes: Faulty purge control valve, wiring short to power.
• P0446 – Evaporative Emission Control System Vent Control Circuit Malfunction
  • causes: Faulty vent control valve, blocked vent valve, wiring or connector.
• P0447 – Evaporative Emission Control System Vent Control Circuit Open
  • causes: Faulty vent control valve, wiring or connector.
• P0448 – Evaporative Emission Control System Vent Control Circuit Shorted
  • causes: Faulty vent control valve, wiring short to power.
• P0449 – Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction
  • causes: Faulty vent valve/solenoid, wiring or connector.

OBD / OBDII DTC P0400 – P0499

• P0450 – Evaporative Emission Control System Pressure Sensor Malfunction
  • causes: Faulty EVAP pressure sensor, wiring or connector.
• P0451 – Evaporative Emission Control System Pressure Sensor Range/Performance
  • causes: Faulty sensor, wiring issues, EVAP system leaks.
• P0452 – Evaporative Emission Control System Pressure Sensor Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0453 – Evaporative Emission Control System Pressure Sensor High Input
  • causes: Short to power, faulty sensor, wiring.
• P0454 – Evaporative Emission Control System Pressure Sensor Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0455 – Evaporative Emission Control System Leak Detected (Gross Leak)
  • causes: Loose or missing fuel cap, cracked or disconnected EVAP hoses, faulty vent or purge solenoids, leaks in the charcoal canister.
• P0456 – Evaporative Emission Control System Leak Detected (Very Small Leak)
  • causes: Tightening or replacing fuel cap, small holes in EVAP hoses, faulty vent or purge solenoids.
• P0457 – Evaporative Emission Control System Leak Detected (Fuel Cap Loose/Off)
  • causes: Loose or missing fuel cap, damaged cap seal, filler neck.
• P0458 – Evaporative Emission Control System Purge Control Valve Circuit Low
  • causes: Short to ground, faulty purge control valve, wiring.
• P0459 – Evaporative Emission Control System Purge Control Valve Circuit High
  • causes: Short to power, faulty purge control valve, wiring.

OBD / OBDII DTC P0400 – P0499

• P0460 – Fuel Level Sensor Circuit Malfunction
  • causes: Faulty fuel level sensor, wiring or connector, ECM/PCM.
• P0461 – Fuel Level Sensor Circuit Range/Performance
  • causes: Faulty fuel level sensor, irregular fuel tank shape or fuel sloshing, wiring.
• P0462 – Fuel Level Sensor Circuit Low Input
  • causes: Short to ground, faulty fuel level sensor, wiring.
• P0463 – Fuel Level Sensor Circuit High Input
  • causes: Short to power, faulty fuel level sensor, wiring.
• P0464 – Fuel Level Sensor Circuit Intermittent
  • causes: Faulty sensor, loose or corroded connectors, wiring.
• P0465 – Purge Flow Sensor Circuit Malfunction
  • causes: Faulty purge flow sensor, wiring or connector.
• P0466 – Purge Flow Sensor Circuit Range/Performance
  • causes: Faulty sensor, wiring.
• P0467 – Purge Flow Sensor Circuit Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0468 – Purge Flow Sensor Circuit High Input
  • causes: Short to power, faulty sensor, wiring.
• P0469 – Purge Flow Sensor Circuit Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.

OBD / OBDII DTC P0400 – P0499

• P0470 – Exhaust Pressure Sensor Malfunction
  • causes: Faulty exhaust pressure sensor, clogged exhaust system, wiring or connector.
• P0471 – Exhaust Pressure Sensor Range/Performance
  • causes: Faulty sensor, clogged exhaust system, wiring.
• P0472 – Exhaust Pressure Sensor Low
  • causes: Short to ground, faulty sensor, wiring.
• P0473 – Exhaust Pressure Sensor High
  • causes: Short to power, faulty sensor, wiring.
• P0474 – Exhaust Pressure Sensor Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0475 – Exhaust Pressure Control Valve Malfunction
  • causes: Faulty exhaust pressure control valve, wiring or connector, clogged exhaust system.
• P0476 – Exhaust Pressure Control Valve Range/Performance
  • causes: Faulty valve, clogged or restricted exhaust system, wiring.
• P0477 – Exhaust Pressure Control Valve Low
  • causes: Short to ground, faulty valve, wiring.
• P0478 – Exhaust Pressure Control Valve High
  • causes: Short to power, faulty valve, wiring.
• P0479 – Exhaust Pressure Control Valve Intermittent
  • causes: Faulty valve, intermittent electrical connections, wiring.

OBD / OBDII DTC P0400 – P0499

• P0480 – Cooling Fan 1 Control Circuit Malfunction
  • causes: Faulty cooling fan motor or relay, wiring or connector, faulty control module.
• P0481 – Cooling Fan 2 Control Circuit Malfunction
  • causes: Same as P0480, but cooling fan 2.
• P0482 – Cooling Fan 3 Control Circuit Malfunction
  • causes: Same as P0480, but cooling fan 3.
• P0483 – Cooling Fan Rationality Check Malfunction
  • causes: Faulty cooling fan motor or controller, sensor malfunction, wiring.
• P0484 – Cooling Fan Circuit Overcurrent
  • causes: Short circuit, faulty fan motor, wiring or connector.
• P0485 – Cooling Fan Power/Ground Circuit Malfunction
  • causes: Faulty fan motor or controller, wiring or connector, poor grounding.
• P0486 – Exhaust Gas Recirculation Sensor ‘B’ Circuit
  • causes: Faulty EGR sensor, wiring or connector, blocked EGR.
• P0487 – Exhaust Gas Recirculation Throttle Position Control Circuit
  • causes: Faulty EGR throttle position controller, wiring, carbon buildup.
• P0488 – Exhaust Gas Recirculation Throttle Position Control Range/Performance
  • causes: Faulty EGR throttle controller, improper calibration, mechanical binding.
• P0489 – Exhaust Gas Recirculation Control Circuit Low
  • causes: Short to ground, faulty EGR valve or solenoid, wiring.

OBD / OBDII DTC P0400 – P0499

• P0490 – Exhaust Gas Recirculation Control Circuit High
  • causes: Short to power, faulty EGR valve or solenoid, wiring issue.
• P0491 – Secondary Air Injection System Insufficient Flow (Bank 1)
  • causes: Faulty air pump, blocked hoses or pipes, faulty one-way check valve.
• P0492 – Secondary Air Injection System Insufficient Flow (Bank 2)
  • causes: Same as P0491, but specific to bank 2.
• P0493 – Fan Overspeed
  • causes: Faulty fan motor or controller, wiring, engine cooling system.
• P0494 – Fan Speed Low
  • causes: Faulty fan motor, wiring, fan control circuit, engine cooling system.
• P0495 – Fan Speed High
  • causes: Faulty fan motor or controller, wiring, engine cooling system.
• P0496 – Evaporative Emission System High Purge Flow
  • causes: Faulty purge valve, stuck open purge valve, EVAP system leak, wiring or connector.
• P0497 – Evaporative Emission System Low Purge Flow
  • causes: Blocked or restricted purge lines, faulty purge valve, EVAP system leak.
• P0498 – Evaporative Emission System Vent Valve Control Circuit Low
  • causes: Short to ground, faulty vent valve, wiring or connector.
• P0499 – Evaporative Emission System Vent Valve Control Circuit High
  • : Short to power, faulty vent valve, wiring or connector.

Why is my Engine Light on?

On-Board Diagnostic (OBD / OBDII) codes are stored within the Engine Control Unit (ECU) or Powertrain Control Module (PCM). These modules store the codes, monitor and control engine performance and emissions. When the system detects a fault or irregularity, it generates a diagnostic trouble code (DTC), which is stored in the ECU/PCM’s memory. This code can be retrieved using an OBD scanner, providing insights into issues.

Are These Codes Causing an Engine Light?

On-Board Diagnostic (OBD or OBDII ) codes are part of a vehicle’s self-diagnostic system.  The ECU and sensors work together to produce DTCs, sometimes resulting in an engine light.  The codes generated do not always cause an engine light.  The development of OBD systems began in the 1980s, driven by increasing environmental concerns and regulations, especially in California. The California Air Resources Board (CARB) played a key role in this, mandating the first OBD systems.  OBD, introduced in the 1980s, was the first system of its kind.  The evolution of OBD technology culminated in the universal OBDII standard in the mid-1990s.  It provided basic capabilities to monitor and diagnose vehicle functions, primarily for emissions control.  OBDII, introduced in the mid-1990s, is a more advanced and standardized system.  This OBDII advanced system was mandatory for all cars sold in the United States from 1996 onwards.  OBDII offers broader diagnostics, real-time data, and standardization across most vehicles.  This made it easier to diagnose and address issues.  OBDII’s standardized diagnostic trouble codes (DTC) and connector types are much better than the earlier OBD system, making it easier to diagnosis and repair.