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 P0600 – P0699

• P0600 – Serial Communication Link Malfunction
  • causes: Faulty wiring or connectors, control modules, communication network.
• P0601 – Internal Control Module Memory Check Sum Error
  • causes: Faulty ECM/PCM, corrupted memory, software.
• P0602 – Control Module Programming Error
  • causes: Programming error, ECM/PCM, software.
• P0603 – Internal Control Module Keep Alive Memory (KAM) Error
  • causes: Faulty ECM/PCM, power supply, battery disconnections.
• P0604 – Internal Control Module Random Access Memory (RAM) Error
  • causes: Faulty ECM/PCM, corrupted RAM, electrical.
• P0605 – Internal Control Module Read Only Memory (ROM) Error
  • causes: Faulty ECM/PCM, corrupted ROM, software.
• P0606 – ECM/PCM Processor
  • causes: Faulty ECM/PCM, software or firmware, electrical.
• P0607 – Control Module Performance
  • causes: Faulty ECM/PCM, software, electrical.
• P0608 – Vehicle Speed Sensor ‘A’
  • causes: Faulty vehicle speed sensor, wiring or connector, ECM/PCM.
• P0609 – Vehicle Speed Sensor ‘B’
  • causes: Same as P0608, but specific to Sensor ‘B’.

OBD / OBDII DTC P0610 – P0619

• P0610 – Control Module Vehicle Options Error
  • causes: Configuration errors, ECM/PCM, software.
• P0611 – Fuel Injector Control Module Performance
  • causes: Faulty Fuel Injector Control Module, electrical issues, connection problems.
• P0612 – Fuel Injector Control Module Relay Control
  • causes: Faulty relay, wiring or connector, control module.
• P0613 – TCM Processor
  • causes: Faulty TCM, electrical, software or firmware.
• P0614 – ECM/TCM Incompatible
  • causes: Incorrect pairing of ECM and TCM, software mismatches, configuration errors.
• P0615 – Starter Relay Circuit
  • causes: Faulty starter relay, wiring or connector issues, ignition switch faults.
• P0616 – Starter Relay Circuit Low
  • causes: Short to ground, faulty starter relay, wiring issue.
• P0617 – Starter Relay Circuit High
  • causes: Short to power, faulty starter relay, wiring issue.
• P0618 – Alternative Fuel Control Module KAM Error
  • Keep Alive Memory (KAM) error in the Alternative Fuel Control Module.
  • causes: Faulty control module, power supply issues, battery disconnections.
• P0619 – Alternative Fuel Control Module RAM/ROM Error
  • causes: Faulty control module, corrupted memory, software issues.

OBD / OBDII DTC P0600 – P0699

• P0620 – Generator Control Circuit Malfunction
  • causes: Faulty alternator, wiring or connector issues, control module.
• P0621 – Generator Lamp ‘L’ Control Circuit Malfunction
  • causes: Faulty alternator, wiring, instrument panel.
• P0622 – Generator Field ‘F’ Control Circuit Malfunction
  • causes: Faulty alternator, control module, wiring or connector.
• P0623 – Generator Lamp Control Circuit
  • causes: Faulty generator lamp, wiring or connector, control module.
• P0624 – Fuel Cap Lamp Control Circuit
  • causes: Faulty fuel cap lamp, wiring or connector, instrument cluster.
• P0625 – Generator Field/F Terminal Circuit Low
  • causes: Faulty alternator, short to ground in wiring, control module.
• P0626 – Generator Field/F Terminal Circuit High
  • causes: Faulty alternator, short to power in wiring, control module.
• P0627 – Fuel Pump ‘A’ Control Circuit/Open
  • causes: Faulty fuel pump relay, wiring or connector, ECM.
• P0628 – Fuel Pump ‘A’ Control Circuit Low
  • causes: Short to ground, faulty fuel pump relay, wiring.
• P0629 – Fuel Pump ‘A’ Control Circuit High
  • causes: Short to power, faulty fuel pump relay, wiring.

OBD / OBDII DTC P0600 – P0699

• P0630 – VIN Not Programmed or Incompatible – ECM/PCM
  • causes: ECM/PCM replacement without VIN programming, software.
• P0631 – VIN Not Programmed or Incompatible – TCM
  • causes: TCM replacement without VIN programming, software.
• P0632 – Odometer Not Programmed – ECM/PCM
  • causes: ECM/PCM replacement without programming, software.
• P0633 – Immobilizer Key Not Programmed – ECM/PCM
  • causes: Replacement of ECM/PCM or immobilizer system components without key programming, immobilizer system faults.
• P0634 – PCM/ECM/TCM Internal Temperature Too High
  • causes: Overheating engine compartment, faulty PCM/ECM/TCM.
• P0635 – Power Steering Control Circuit
  • causes: Faulty power steering control module, wiring or connector.
• P0636 – Power Steering Control Circuit Low
  • causes: Short to ground, faulty power steering control module, wiring.
• P0637 – Power Steering Control Circuit High
  • causes: Short to power, faulty power steering control module, wiring.
• P0638 – Throttle Actuator Control Range/Performance (Bank 1)
  • causes: Faulty throttle body, wiring or connector issues, ECM.
• P0639 – Throttle Actuator Control Range/Performance (Bank 2)
  • causes: Same as P0638, but bank 2, including faulty throttle body, wiring or connector, and ECM.

OBD / OBDII DTC P0600 – P0699

• P0640 – Intake Air Heater Control Circuit
  • causes: Faulty intake air heater, wiring or connector issues, control module.
• P0641 – Sensor Reference Voltage ‘A’ Circuit/Open
  • causes: Electrical, faulty sensors or ECM, wiring.
• P0642 – Sensor Reference Voltage ‘A’ Circuit Low
  • causes: Short to ground, electrical, faulty sensors or ECM.
• P0643 – Sensor Reference Voltage ‘A’ Circuit High
  • causes: Short to power, electrical, faulty sensors or ECM.
• P0644 – Driver Display Serial Communication Circuit
  • causes: Faulty instrument panel, wiring or connector, control module.
• P0645 – A/C Clutch Relay Control Circuit
  • causes: Faulty A/C clutch relay, wiring or connector issues, control module.
• P0646 – A/C Clutch Relay Control Circuit Low
  • causes: Short to ground, faulty A/C clutch relay, wiring.
• P0647 – A/C Clutch Relay Control Circuit High
  • causes: Short to power, faulty A/C clutch relay, wiring.
• P0648 – Immobilizer Lamp Control Circuit
  • causes: Faulty immobilizer lamp, wiring or connector, control module.
• P0649 – Speed Control Lamp Control Circuit
  • causes: Faulty speed control lamp, wiring or connector, control module.

OBD / OBDII DTC P0600 – P0699

• P0650 – Malfunction Indicator Lamp (MIL) Control Circuit Malfunction
  • causes: Faulty MIL, wiring or connector, ECM/PCM.
• P0651 – Sensor Reference Voltage ‘B’ Circuit/Open
  • causes: Same as P0641, but voltage ‘B’.
• P0652 – Sensor Reference Voltage ‘B’ Circuit Low
  • causes: Same as P0642, but voltage ‘B’.
• P0653 – Sensor Reference Voltage ‘B’ Circuit High
  • causes: Same as P0643, but voltage ‘B’.
• P0654 – Engine RPM Output Circuit
  • causes: RPM signal output, wiring or connector, ECM/PCM.
• P0655 – Engine Hot Lamp Output Control Circuit
  • causes: engine temperature lamp, wiring or connector, ECM/PCM.
• P0656 – Fuel Level Output Circuit
  • causes: fuel level sensor, wiring or connector, ECM/PCM.
• P0657 – Actuator Supply Voltage ‘A’ Circuit/Open
  • causes: Electrical, actuators, wiring or connector.
• P0658 – Actuator Supply Voltage ‘A’ Circuit Low
  • causes: Short to ground, electrical, actuators.
• P0659 – Actuator Supply Voltage ‘A’ Circuit High
  • causes: Short to power, electrical, actuators.

OBD / OBDII DTC P0600 – P0699

• P0660 – Intake Manifold Tuning Valve Control Circuit/Open (Bank 1)
  • causes: Faulty intake manifold tuning valve, wiring or connector, ECM/PCM.
• P0661 – Intake Manifold Tuning Valve Control Circuit Low (Bank 1)
  • causes: Short to ground, faulty intake manifold tuning valve, wiring.
• P0662 – Intake Manifold Tuning Valve Control Circuit High (Bank 1)
  • causes: Short to power, faulty intake manifold tuning valve, wiring.
• P0663 – Intake Manifold Tuning Valve Control Circuit/Open (Bank 2)
  • causes: Same as P0660, but specific to bank 2.
• P0664 – Intake Manifold Tuning Valve Control Circuit Low (Bank 2)
  • causes: Same as P0661, but specific to bank 2.
• P0665 – Intake Manifold Tuning Valve Control Circuit High (Bank 2)
  • causes: Same as P0662, but specific to bank 2.
• P0666 – Intake Manifold Tuning Valve Sensor Circuit
  • causes: Faulty sensor, wiring or connector, ECM/PCM.
• P0667 – Intake Manifold Tuning Valve Sensor Circuit Low
  • causes: Short to ground, sensor, wiring.
• P0668 – Intake Manifold Tuning Valve Sensor Circuit High
  • causes: Short to power, sensor, wiring.
• P0669 – Intake Manifold Tuning Valve Sensor Range/Performance
  • causes: sensor, mechanical problems with the intake manifold, wiring or connector.

OBD / OBDII DTC P0600 – P0699

• P0670 – Glow Plug Module Control Circuit
  • causes: glow plug module, wiring or connector, ECM/PCM.
• P0671 – Cylinder 1 Glow Plug Circuit
  • causes: glow plug, wiring or connector, control module.
• P0672 – Cylinder 2 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 2.
• P0673 – Cylinder 3 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 3.
• P0674 – Cylinder 4 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 4.
• P0675 – Cylinder 5 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 5.
• P0676 – Cylinder 6 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 6.
• P0677 – Cylinder 7 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 7.
• P0678 – Cylinder 8 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 8.
• P0679 – Cylinder 9 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 9.

OBD / OBDII DTC P0600 – P0699

• P0680 – Cylinder 10 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 10.
• P0681 – Cylinder 11 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 11.
• P0682 – Cylinder 12 Glow Plug Circuit
  • causes: Same as P0671, but cylinder 12.
• P0683 – Glow Plug Control Module to PCM Communication Circuit
  • causes: glow plug control module, wiring or connector, PCM.
• P0684 – Glow Plug Control Module to PCM Communication Circuit Range/Performance
  • causes: Same as P0683.
• P0685 – ECM/PCM Power Relay Control Circuit/Open
  • causes: ECM/PCM power relay, wiring or connector, ECM/PCM.
• P0686 – ECM/PCM Power Relay Control Circuit Low
  • causes: Short to ground,  ECM/PCM power relay, wiring.
• P0687 – ECM/PCM Power Relay Control Circuit High
  • causes: Short to power, ECM/PCM power relay, wiring.
• P0688 – ECM/PCM Power Relay Sense Circuit/Open
  • causes: Faulty ECM/PCM power relay, wiring or connector, ECM/PCM.
• P0689 – ECM/PCM Power Relay Sense Circuit Low
  • causes: Short to ground, faulty ECM/PCM power relay, wiring.

OBD / OBDII DTC P0600 – P0699

• P0690 – ECM/PCM Power Relay Sense Circuit High
  • causes: Short to power, faulty ECM/PCM power relay, wiring issue.
• P0691 – Fan 1 Control Circuit Low
  • causes: Short to ground, faulty fan control relay, wiring issue.
• P0692 – Fan 1 Control Circuit High
  • causes: Short to power, faulty fan control relay, wiring issue.
• P0693 – Fan 2 Control Circuit Low
  • causes: Same as P0691, but specific to fan 2.
• P0694 – Fan 2 Control Circuit High
  • causes: Same as P0692, but specific to fan 2.
• P0695 – Fan 3 Control Circuit Low
  • causes: Similar to P0691, but specific to fan 3.
• P0696 – Fan 3 Control Circuit High
  • causes: Similar to P0692, but specific to fan 3, including short to power, faulty fan control relay, or wiring issues.
• P0697 – Sensor Reference Voltage ‘C’ Circuit/Open
  • causes: Electrical issues, faulty sensors or control modules, wiring problems.
• P0698 – Sensor Reference Voltage ‘C’ Circuit Low
  • causes: Short to ground, electrical issues, faulty sensors or control modules.
• P0699 – Sensor Reference Voltage ‘C’ Circuit High
  • causes: Short to power, electrical issues, faulty sensors or control modules.

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.