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 P0100 – P0109

• P0100 – Mass or Volume Air Flow Circuit Malfunction
  • causes: Faulty MAF sensor, poor electrical connection, air intake leak.
• P0101 – Mass or Volume Air Flow Circuit Range/Performance Problem
  • causes: Dirty or faulty MAF sensor, air leak, wiring.
• P0102 – Mass or Volume Air Flow Circuit Low Input
  • causes: Faulty MAF sensor, short circuit, air intake leak.
• P0103 – Mass or Volume Air Flow Circuit High Input
  • causes: Faulty MAF sensor, short circuit, wiring.
• P0104 – Mass or Volume Air Flow Circuit Intermittent
  • causes: Loose or corroded connectors, faulty MAF sensor, wiring.
• P0105 – Manifold Absolute Pressure/Barometric Pressure Circuit Malfunction
  • causes: Faulty MAP sensor, wiring or connector, vacuum leaks.
• P0106 – Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem
  • causes: Faulty MAP sensor, vacuum leak, electrical connection.
• P0107 – Manifold Absolute Pressure/Barometric Pressure Circuit Low Input
  • causes: Short to ground, faulty MAP sensor, vacuum leak.
• P0108 – Manifold Absolute Pressure/Barometric Pressure Circuit High Input
  • causes: Short to power, faulty MAP sensor, wiring.
• P0109 – Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent
  • causes: Faulty MAP sensor, loose or corroded connectors, wiring.

OBD / OBDII DTC P0110 – P0119

• P0110 – Intake Air Temperature Circuit Malfunction
  • causes: Faulty IAT sensor, poor electrical connection, wiring.
• P0111 – Intake Air Temperature Circuit Range/Performance Problem
  • causes: Dirty or faulty IAT sensor, wiring problem, intake system leak.
• P0112 – Intake Air Temperature Circuit Low Input
  • causes: Short to ground, faulty IAT sensor, wiring.
• P0113 – Intake Air Temperature Circuit High Input
  • causes: Short to power, faulty IAT sensor, wiring.
• P0114 – Intake Air Temperature Circuit Intermittent
  • causes: Faulty IAT sensor, loose or corroded connectors, wiring.
• P0115 – Engine Coolant Temperature Circuit Malfunction
  • causes: Faulty ECT sensor, wiring or connector issue, coolant system fault.
• P0116 – Engine Coolant Temperature Circuit Range/Performance Problem
  • causes: Faulty ECT sensor, coolant system, wiring.
• P0117 – Engine Coolant Temperature Circuit Low Input
  • causes: Short to ground, faulty ECT sensor, wiring.
• P0118 – Engine Coolant Temperature Circuit High Input
  • causes: Short to power, faulty ECT sensor, wiring.
• P0119 – Engine Coolant Temperature Circuit Intermittent
  • causes: Faulty ECT sensor, loose or corroded connectors

OBD / OBDII DTC P0120 – P0129

• P0120 – Throttle/Pedal Position Sensor/Switch ‘A’ Circuit Malfunction
  • causes: Faulty throttle position sensor, poor electrical connection, wiring.
• P0121 – Throttle/Pedal Position Sensor/Switch ‘A’ Circuit Range/Performance Problem
  • causes: Worn throttle position sensor, electrical connection, throttle body.
• P0122 – Throttle/Pedal Position Sensor/Switch ‘A’ Circuit Low Input
  • causes: Short to ground, faulty throttle position sensor, wiring.
• P0123 – Throttle/Pedal Position Sensor/Switch ‘A’ Circuit High Input
  • causes: Short to power, faulty throttle position sensor, wiring.
• P0124 – Throttle/Pedal Position Sensor/Switch ‘A’ Circuit Intermittent
  • causes: Faulty throttle position sensor, loose or corroded connectors, wiring.
• P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control
  • causes: Low coolant level, faulty thermostat, malfunctioning ECT sensor.
• P0126 – Insufficient Coolant Temperature for Stable Operation
  • causes: Same as P0125.
• P0127 – Intake Air Temperature Too High
  • causes: Faulty IAT sensor, heat soak effect, air intake.
• P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
  • causes: Stuck open thermostat, faulty ECT sensor.
• P0129 – Barometric Pressure Too Low
  • causes: Faulty barometric pressure sensor, weather-related conditions.

OBD / OBDII DTC P0130 – P0139

• P0130 – O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
  • causes: Faulty oxygen sensor, wiring or connector, exhaust leak.
• P0131 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0132 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0133 – O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.
• P0134 – O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0135 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.
• P0136 – O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
  • Same as P0130.
• P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0139 – O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.

OBD / OBDII DTC P0140 – P0149

• P0140 – O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0141 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.
• P0142 – O2 Sensor Circuit Malfunction (Bank 1 Sensor 3)
  • causes: Faulty oxygen sensor, wiring or connector, exhaust leak.
• P0143 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 3)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0144 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 3)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0145 – O2 Sensor Circuit Slow Response (Bank 1 Sensor 3)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.
• P0146 – O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 3)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0147 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 3)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.
• P0148 – Fuel Delivery Error
  • causes: Faulty fuel pump, clogged fuel filter, injector, fuel pressure.

OBD / OBDII DTC P0150 – P0159

• P0150 – O2 Sensor Circuit Malfunction (Bank 2 Sensor 1)
  • causes: Faulty oxygen sensor, wiring or connector, exhaust leak.
• P0151 – O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0152 – O2 Sensor Circuit High Voltage (Bank 2 Sensor 1)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0153 – O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.
• P0154 – O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0155 – O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.
• P0156 – O2 Sensor Circuit Malfunction (Bank 2 Sensor 2)
  • causes: Faulty oxygen sensor, wiring or connector, exhaust leak.
• P0157 – O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0158 – O2 Sensor Circuit High Voltage (Bank 2 Sensor 2)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0159 – O2 Sensor Circuit Slow Response (Bank 2 Sensor 2)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.

OBD / OBDII DTC P0160 – P0169

• P0160 – O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0161 – O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.
• P0162 – O2 Sensor Circuit Malfunction (Bank 2 Sensor 3)
  • causes: Faulty oxygen sensor, wiring or connector, exhaust leak.
• P0163 – O2 Sensor Circuit Low Voltage (Bank 2 Sensor 3)
  • causes: Short to ground, faulty oxygen sensor, wiring.
• P0164 – O2 Sensor Circuit High Voltage (Bank 2 Sensor 3)
  • causes: Short to power, faulty oxygen sensor, wiring.
• P0165 – O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)
  • causes: Aging or contaminated oxygen sensor, exhaust leak, wiring.
• P0166 – O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)
  • causes: Faulty oxygen sensor, wiring or connector.
• P0167 – O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 3)
  • causes: Faulty oxygen sensor heater, wiring or connector, fuse.

OBD / OBDII DTC P0170 – P0179

• P0170 – Fuel Trim Malfunction (Bank 1)
  • causes: Vacuum leaks, faulty fuel injectors, MAF sensor or O2 sensor.
• P0171 – System Too Lean (Bank 1)
  • causes: Vacuum leaks, clogged fuel injectors, faulty MAF or O2 sensors.
• P0172 – System Too Rich (Bank 1)
  • causes: Leaking fuel injectors, faulty MAF or O2 sensors, excessive fuel pressure.
• P0173 – Fuel Trim Malfunction (Bank 2)
  • causes: Same as P0170.
• P0174 – System Too Lean (Bank 2)
  • causes: Vacuum leaks, clogged fuel injectors, faulty MAF or O2 sensors.
• P0175 – System Too Rich (Bank 2)
  • causes: Leaking fuel injectors, faulty MAF or O2 sensors, excessive fuel pressure.
• P0176 – Fuel Composition Sensor Circuit Malfunction
  • causes: Faulty fuel composition sensor, wiring or connector.
• P0177 – Fuel Composition Sensor Circuit Range/Performance
  • causes: Faulty sensor, wiring or connector.
• P0178 – Fuel Composition Sensor Circuit Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0179 – Fuel Composition Sensor Circuit High Input
  • causes: Short to power, faulty sensor, wiring.

OBD / OBDII DTC P0180 – P0189

• P0180 – Fuel Temperature Sensor ‘A’ Circuit Malfunction
  • causes: Faulty fuel temperature sensor, wiring or connector.
• P0181 – Fuel Temperature Sensor ‘A’ Circuit Range/Performance
  • causes: Faulty sensor, wiring or connector.
• P0182 – Fuel Temperature Sensor ‘A’ Circuit Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0183 – Fuel Temperature Sensor ‘A’ Circuit High Input
  • causes: Short to power, faulty sensor, wiring.
• P0184 – Fuel Temperature Sensor ‘A’ Circuit Intermittent
  • causes: Faulty sensor, loose or corroded connectors, wiring.
• P0185 – Fuel Temperature Sensor ‘B’ Circuit Malfunction
  • causes: Faulty fuel temperature sensor, wiring or connector.
• P0186 – Fuel Temperature Sensor ‘B’ Circuit Range/Performance
  • causes: Faulty sensor, wiring or connector.
• P0187 – Fuel Temperature Sensor ‘B’ Circuit Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0188 – Fuel Temperature Sensor ‘B’ Circuit High Input
  • causes: Short to power, faulty sensor, wiring.
• P0189 – Fuel Temperature Sensor ‘B’ Circuit Intermittent
  • causes: Faulty sensor, loose or corroded connectors, wiring.

OBD / OBDII DTC P0190 – P0199

• P0190 – Fuel Rail Pressure Sensor Circuit Malfunction
  • causes: Faulty fuel rail pressure sensor, wiring or connector.
• P0191 – Fuel Rail Pressure Sensor Circuit Range/Performance
  • causes: Faulty sensor, wiring or connector, fuel pressure.
• P0192 – Fuel Rail Pressure Sensor Circuit Low Input
  • causes: Short to ground, faulty sensor, wiring.
• P0193 – Fuel Rail Pressure Sensor Circuit High Input
  • causes: Short to power, faulty sensor, wiring.
• P0194 – Fuel Rail Pressure Sensor Circuit Intermittent
  • causes: Faulty sensor, loose or corroded connectors, wiring, intermittent fuel pressure.
• P0195 – Engine Oil Temperature Sensor Malfunction
  • causes: Faulty engine oil temperature sensor, wiring or connector.
• P0196 – Engine Oil Temperature Sensor Range/Performance
  • causes: Faulty sensor, wiring or connector.
• P0197 – Engine Oil Temperature Sensor Low
  • causes: Short to ground, faulty sensor, wiring.
• P0198 – Engine Oil Temperature Sensor High
  • causes: Short to power, faulty sensor, wiring.
• P0199 – Engine Oil Temperature Sensor Intermittent
  • causes: Faulty sensor, loose or corroded connectors, wiring.

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.