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 P0900 – P0999

• P0900 – Clutch Actuator Circuit/Open
  • causes: Faulty clutch actuator, wiring or connector issues, transmission control module (TCM).
• P0901 – Clutch Actuator Circuit Range/Performance
  • causes: Faulty clutch actuator, incorrect actuator adjustment, wiring.
• P0902 – Clutch Actuator Circuit Low
  • causes: Short to ground, faulty clutch actuator, wiring.
• P0903 – Clutch Actuator Circuit High
  • causes: Short to power, faulty clutch actuator, wiring.
• P0904 – Clutch Position Sensor Circuit
  • causes: Faulty clutch position sensor, wiring or connector issues, ECM or TCM.
• P0905 – Clutch Position Sensor Circuit Range/Performance
  • causes: Faulty sensor, wiring problems, ECM or TCM.
• P0906 – Clutch Position Sensor Circuit Low
  • causes: Short to ground, faulty sensor, wiring.
• P0907 – Clutch Position Sensor Circuit High
  • causes: Short to power, faulty sensor, wiring.
• P0908 – Clutch Position Sensor Circuit Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0909 – Clutch Position Calibration Fault
  • causes: Incorrect clutch position sensor calibration, faulty sensor, TCM.

OBD / OBDII DTC P0900 – P0999

• P0910 – Clutch Position Control Error
  • causes: Mechanical failure in the clutch assembly, faulty clutch position sensor, TCM.
• P0911 – Clutch Position Actuator Circuit/Open
  • causes: Faulty clutch position actuator, wiring or connector.
• P0912 – Clutch Position Actuator Circuit Low
  • causes: Short to ground, faulty actuator, wiring.
• P0913 – Clutch Position Actuator Circuit High
  • causes: Short to power, faulty actuator, wiring.
• P0914 – Gear Shift Position Circuit
  • causes: Faulty gear shift position sensor, wiring or connector issues, transmission control module (TCM).
• P0915 – Gear Shift Position Circuit Range/Performance
  • causes: Faulty sensor, wiring problems, TCM.
• P0916 – Gear Shift Position Circuit Low
  • causes: Short to ground, faulty sensor, wiring.
• P0917 – Gear Shift Position Circuit High
  • causes: Short to power, faulty sensor, wiring.
• P0918 – Gear Shift Position Circuit Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0919 – Gear Shift Position Control Error
  • causes: Mechanical failure in the gear shift system, faulty gear shift position sensor, TCM.

OBD / OBDII DTC P0900 – P0999

• P0920 – Gear Shift Forward Actuator Circuit/Open
  • causes: Faulty gear shift forward actuator, wiring or connector.
• P0921 – Gear Shift Forward Actuator Circuit Range/Performance
  • causes: Faulty gear shift forward actuator, wiring problems, transmission control module (TCM).
• P0922 – Gear Shift Forward Actuator Circuit Low
  • causes: Short to ground, faulty actuator, wiring.
• P0923 – Gear Shift Forward Actuator Circuit High
  • causes: Short to power, faulty actuator, wiring.
• P0924 – Gear Shift Reverse Actuator Circuit/Open
  • causes: Faulty gear shift reverse actuator, wiring or connector.
• P0925 – Gear Shift Reverse Actuator Circuit Range/Performance
  • causes: Faulty gear shift reverse actuator, wiring problems, TCM.
• P0926 – Gear Shift Reverse Actuator Circuit Low
  • causes: Short to ground, faulty actuator, wiring.
• P0927 – Gear Shift Reverse Actuator Circuit High
  • causes: Short to power, faulty actuator, wiring.
• P0928 – Gear Shift Lock Solenoid Control Circuit/Open
  • causes: Faulty gear shift lock solenoid, wiring or connector.
• P0929 – Gear Shift Lock Solenoid Control Circuit Range/Performance
  • causes: Faulty gear shift lock solenoid, wiring.

OBD / OBDII DTC P0900 – P0999

• P0930 – Gear Shift Lock Solenoid Control Circuit Low
  • causes: Short to ground, faulty solenoid, wiring.
• P0931 – Gear Shift Lock Solenoid Control Circuit High
  • causes: Short to power, faulty solenoid, wiring.
• P0932 – Hydraulic Pressure Sensor Circuit
  • causes: Faulty hydraulic pressure sensor, wiring or connector issues, hydraulic system.
• P0933 – Hydraulic Pressure Sensor Range/Performance
  • causes: Faulty sensor, wiring problems, hydraulic system.
• P0934 – Hydraulic Pressure Sensor Circuit Low
  • causes: Short to ground, faulty sensor, wiring.
• P0935 – Hydraulic Pressure Sensor Circuit High
  • causes: Short to power, faulty sensor, wiring.
• P0936 – Hydraulic Pressure Sensor Circuit Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0937 – Hydraulic Oil Temperature Sensor Circuit
  • causes: Faulty hydraulic oil temperature sensor, wiring or connector issues, hydraulic system.
• P0938 – Hydraulic Oil Temperature Sensor Range/Performance
  • causes: Faulty sensor, wiring problems, hydraulic system.
• P0939 – Hydraulic Oil Temperature Sensor Circuit Low
  • causes: Short to ground, faulty sensor, wiring issue.

OBD / OBDII DTC P0900 – P0999

• P0940 – Hydraulic Oil Temperature Sensor Circuit High
  • causes: Short to power, faulty sensor, wiring.
• P0941 – Hydraulic Oil Temperature Sensor Circuit Intermittent
  • causes: Faulty sensor, intermittent electrical connections, wiring.
• P0942 – Hydraulic Pressure Unit
  • causes: Faulty hydraulic pressure unit, hydraulic system faults, wiring or connector.
• P0943 – Hydraulic Pressure Unit Cycling Period Too Short
  • causes: Hydraulic system faults, faulty hydraulic pressure unit, control module.
• P0944 – Hydraulic Pressure Unit Loss of Pressure
  • causes: Hydraulic fluid leaks, faulty hydraulic pressure unit, hydraulic system.
• P0945 – Hydraulic Pump Relay Circuit/Open
  • causes: Faulty hydraulic pump relay, wiring or connector.
• P0946 – Hydraulic Pump Relay Circuit Range/Performance
  • causes: Faulty hydraulic pump relay, wiring.
• P0947 – Hydraulic Pump Relay Circuit Low
  • causes: Short to ground, faulty relay, wiring.
• P0948 – Hydraulic Pump Relay Circuit High
  • causes: Short to power, faulty relay, wiring.
• P0949 – Adaptive Learning Not Done
  • causes: Recent transmission repair or replacement, transmission control module (TCM).

OBD / OBDII DTC P0900 – P0999

• P0950 – Auto Shift Manual Control Circuit
  • causes: Faulty auto shift manual switch, wiring or connector issues, TCM.
• P0951 – Auto Shift Manual Control Circuit Range/Performance
  • causes: Faulty switch, wiring problems, TCM.
• P0952 – Auto Shift Manual Control Circuit Low
  • causes: Short to ground, faulty switch, wiring.
• P0953 – Auto Shift Manual Control Circuit High
  • causes: Short to power, faulty switch, wiring.
• P0954 – Auto Shift Manual Control Circuit Intermittent
  • causes: Intermittent electrical connections, faulty switch, wiring.
• P0955 – Auto Shift Manual Mode Circuit
  • causes: Faulty auto shift manual mode switch, wiring or connector issues, TCM.
• P0956 – Auto Shift Manual Mode Circuit Range/Performance
  • causes: Faulty switch, wiring problems, TCM.
• P0957 – Auto Shift Manual Mode Circuit Low
  • causes: Short to ground, faulty switch, wiring.
• P0958 – Auto Shift Manual Mode Circuit High
  • causes: Short to power, faulty switch, wiring.
• P0959 – Auto Shift Manual Mode Circuit Intermittent
  • causes: Intermittent electrical connections, faulty switch, wiring.

OBD / OBDII DTC P0900 – P0999

• P0960 – Pressure Control Solenoid ‘A’ Control Circuit/Open
  • causes: Faulty pressure control solenoid, wiring or connector issues, TCM.
• P0961 – Pressure Control Solenoid ‘A’ Control Circuit Range/Performance
  • causes: Faulty solenoid, wiring problems, TCM.
• P0962 – Pressure Control Solenoid ‘A’ Control Circuit Low
  • causes: Short to ground, faulty pressure control solenoid, wiring.
• P0963 – Pressure Control Solenoid ‘A’ Control Circuit High
  • causes: Short to power, faulty pressure control solenoid, wiring.
• P0964 – Pressure Control Solenoid ‘B’ Control Circuit/Open
  • causes: Faulty pressure control solenoid, wiring or connector issues, transmission control module (TCM).
• P0965 – Pressure Control Solenoid ‘B’ Control Circuit Range/Performance
  • causes: Faulty solenoid, wiring problems, TCM.
• P0966 – Pressure Control Solenoid ‘B’ Control Circuit Low
  • causes: Short to ground, faulty pressure control solenoid, wiring.
• P0967 – Pressure Control Solenoid ‘B’ Control Circuit High
  • causes: Short to power, faulty pressure control solenoid, wiring.
• P0968 – Pressure Control Solenoid ‘C’ Control Circuit/Open
  • causes: Faulty pressure control solenoid, wiring or connector issues, TCM.
• P0969 – Pressure Control Solenoid ‘C’ Control Circuit Range/Performance
  • causes: Faulty solenoid, wiring problems, TCM.

OBD / OBDII DTC P0900 – P0999

• P0970 – Pressure Control Solenoid ‘C’ Control Circuit Low
  • causes: Short to ground, faulty pressure control solenoid, wiring.
• P0971 – Pressure Control Solenoid ‘C’ Control Circuit High
  • causes: Short to power, faulty pressure control solenoid, wiring.
• P0972 – Shift Solenoid ‘A’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, TCM.
• P0973 – Shift Solenoid ‘A’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0974 – Shift Solenoid ‘A’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0975 – Shift Solenoid ‘B’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, TCM.
• P0976 – Shift Solenoid ‘B’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0977 – Shift Solenoid ‘B’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0978 – Shift Solenoid ‘C’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, TCM.
• P0979 – Shift Solenoid ‘C’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.

OBD / OBDII DTC P0900 – P0999

• P0980 – Shift Solenoid ‘C’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0981 – Shift Solenoid ‘D’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, transmission control module (TCM).
• P0982 – Shift Solenoid ‘D’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0983 – Shift Solenoid ‘D’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0984 – Shift Solenoid ‘E’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, TCM.
• P0985 – Shift Solenoid ‘E’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0986 – Shift Solenoid ‘E’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0987 – Transmission Fluid Pressure Sensor/Switch ‘E’ Circuit
  • causes: Faulty sensor/switch, wiring or connector, hydraulic problems in the transmission.
• P0988 – Transmission Fluid Pressure Sensor/Switch ‘E’ Circuit Range/Performance
  • causes: Same as P0987.
• P0989 – Transmission Fluid Pressure Sensor/Switch ‘E’ Circuit Low
  • causes: Short to ground, faulty sensor/switch, wiring.

OBD / OBDII DTC P0900 – P0999

• P0990 – Transmission Fluid Pressure Sensor/Switch ‘E’ Circuit High
  • causes: Short to power, faulty sensor/switch, wiring.
• P0991 – Transmission Fluid Pressure Sensor/Switch ‘F’ Circuit
  • causes: Faulty sensor/switch, wiring or connector issues, hydraulic problems in the transmission.
• P0992 – Transmission Fluid Pressure Sensor/Switch ‘F’ Circuit Range/Performance
  • causes: Same as P0991.
• P0993 – Transmission Fluid Pressure Sensor/Switch ‘F’ Circuit Low
  • causes: Short to ground, faulty sensor/switch, wiring.
• P0994 – Transmission Fluid Pressure Sensor/Switch ‘F’ Circuit High
  • causes: Short to power, faulty sensor/switch, wiring.
• P0995 – Shift Solenoid ‘F’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, TCM.
• P0996 – Shift Solenoid ‘F’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0996 – Shift Solenoid ‘F’ Control Circuit Low
  • causes: Short to ground, faulty shift solenoid, wiring.
• P0997 – Shift Solenoid ‘F’ Control Circuit High
  • causes: Short to power, faulty shift solenoid, wiring.
• P0998 – Shift Solenoid ‘F’ Control Circuit Intermittent
  • causes: Intermittent electrical connections, faulty shift solenoid, wiring.
• P0999 – Shift Solenoid ‘F’ Control Circuit Range/Performance
  • causes: Faulty shift solenoid, wiring problems, transmission control module (TCM).

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.

What Are These Codes in my ECU?

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.