Common Generic OBD II (P0) Codes

The Diagnostic Trouble Codes (DTCs) listed below are some of the most commonly reported on OBD II vehicles, those 1996 and newer.
Please note that these DTC definitions are provided for reference only. We recommend that you consult a service manual for your vehicle before actually attempting repairs.
To order a book containing a complete list of Generic and Enhanced codes for GM, Ford and Chrysler or Asian OBD II vehicles, please visit

P0101 - MAF Sensor Performance
This code indicates that your Mass AirFlow sensor is not responding properly or is out of its signal range. Be careful about replacing sensors without testing them according to proper procedures. Many times sensor related codes are caused by poor continuity in the wiring or connections. Some of them can be contaminated by dirt and simply need to be cleaned.

P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
Replacing O2 sensor fixed it.
P0143 O2 Sensor Circuit, Bank 1 Sensor 3
This refers to the electrical circuit that controls the oxygen sensor behind the catalytic converter. This could indicate a problem with the sensor or the wiring leading to it. Consult a service manual before replacing sensors

P0171, P0174, P0175 Rich or Lean Conditions
Although the O2 sensors are responsible for reporting the air/fuel mixture, they are seldom responsible for an overly rich or overly lean condition.

P0171 and P0174 indicate the engine is running on an air/fuel mixture that is excessively lean. This can be caused by several different problems including low fuel pressure, vacuum leaks, exhaust leaks, misfires, clogged fuel injectors, MAF sensor problems, and others.

P0174 indicates that the fuel control portion of the engine's computer is correcting for an excessively lean air/fuel mixture. Some common causes for this include incorrect fuel pressure, an EGR system malfunction, an MAF sensor malfunction, intake or vacuum leaks, incorrect cam timing, a bad PCV system, or exhaust leaks.

P0175 indicates Bank 2 of the engine is running too rich. This simply means that the bank of cylinders opposite the bank containing cylinder #1 is receiving too much fuel or not enough air.

P0300 Random Cylinder Misfires Detected
Faulty spark plugs, plug wires, distributor cap, rotor, ignition coil or fuel injection system problems can all cause this code.
See misfire.
P0301-P0308 Cylinder 1-8 Misfire Detected
Faulty spark plugs, plug wires, distributor cap, rotor, ignition coil or fuel injection system problems can all cause this code.

P0410 Secondary Air Injection System
"Secondary Air" is an abbreviated term for the "Secondary Air Injection System". This system uses a separate air pump and a series of metal and rubber tubes to force outside air into the exhaust stream and help the catalytic converter burn away excessive emissions.
Possible causes and fixes are:

  • Poor connection, rubbed-through wire insulation or a broken wire inside the insulation.
  • Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness.
  • Check for worn or loose AIR pump drive belt.
  • Check for pinched, kinked or restricted AIR pipes, hoses or fittings.

    Problems with this system can stem from electrical circuits, solenoids, or switches that control the pump and valves. Other problems include sticking valves, loose, rusted, or poorly connected lines, a broken drive belt, or possibly a bad pump.

    P0102 Mass Air Flow (MAF) Performance Conditions
    Powertrain Control Module (PCM) detected that the MAF input was out of range.
  • Check for faulty connections or damaged harness.
  • Ensure harness is not routed too close to high-voltage wires, such as spark plug wires.
  • Check for a plugged intake air duct or filter element
  • Check minimum air rate.
  • Check for plugged intake air duct or filter element.

    P0440 EVAP System Malfunction
    Also P0442 and P0456 EVAP leak
    P0440 is defined as EVAP system incorrect purge flow. A problem anywhere in the EVAP system can cause an "incorrect purge flow". This simply means that the computer has detected that the fuel vapors are not flowing the way they should.

    The EVAP system uses the engine's vacuum to periodically draw fuel vapors out of the tank through a hose and stores these vapors in a charcoal filled canister. These vapors are eventually sent through another hose to be burned inside the engine. The engine computer controls this system by monitoring the pressure inside the fuel tank, and opening an electronic valve to allow the vapor to flow or "purge".

    A good service manual for your vehicle should give you a complete description of your particular EVAP system, as well as some ideas on how to test it. The problem could be as simple as a loose or damaged hose connection, or even a loose gas cap. Thorough visual inspections of the system reveal any obvious problems.
    See EVAP codes.

    P0401 (EGR Flow Insufficient)
    The EGR (Exhaust Gas Recirculation) system causes some of the exhaust gas to be redirected back into the intake manifold. The idea is to reduce the combustion temperature and reduce pollution. As near as I understand it, there is a EGR valve, EGR solenoid, EGR flow sensor (called a Differential Pressure Feedback (DPF) EGR sensor.

    Look for:

    • EGR valve stuck closed
    • EGR valve leaks vacuum
    • EGR flow path restricted
    • EGRVR circuit shorted to PWR
    • VREF open to D.P.F. EGR sensor
    • DPF EGR sensor downstream hose off or plugged
    • Defective DPF EGR sensor
    • EGRVR circuit open to PCM
    • VPWR open to EGRVR solenoid
    • D.P.F. EGR sensor hoses both off
    • D.P.F. EGR sensor hoses reversed
    • Damaged EGR orifice tube
    • Damaged EGRVR solenoid
    • Damaged PCM (Computer)
    • Perform KOER self-test and look for DTC P1408 as an indication of a hard fault. If P1408 is not present, look for contamination, restrictions, leaks, and intermittents.
    • Bad Vacume line
    See details for P0401.

    last updated 16 Mar 2005