SAE J2012 / ISO 15031-6
OBD-II Codes — Symptoms, Root Causes & Diagnostic Steps
Standardized OBD-II Diagnostic Trouble Codes (DTC) with freeze-frame interpretation, root causes, and step-by-step multimeter/scanner checks.
| P0xxx / P2xxx — Powertrain | Engine, fuel injection, ignition, emissions (catalytic converter, EVAP, EGR), and transmission |
|---|---|
| C0xxx — Chassis | ABS wheel speed sensors, Electronic Stability Control (ESC/ESP), steering angle, and suspension |
| B0xxx / B1xxx — Body | Airbag / Supplemental Restraint System (SRS), Body Control Module (BCM), HVAC, and immobilizer |
| U0xxx — Network (CAN-Bus) | High-speed / low-speed CAN communication between ECM, TCM, ABS, and gateway modules |
Catalyst System Efficiency Below Threshold (Bank 1)
Oxygen sensor 2 (downstream) waveform mirrors upstream O2 sensor, indicating reduced oxygen storage capacity in the three-way catalytic converter.
System Too Lean (Bank 1)
ECM has added more than +20% to +25% combined Short-Term and Long-Term Fuel Trim on Bank 1 to maintain stoichiometric λ = 1.0.
System Too Lean (Bank 2)
Excessive positive fuel trim correction on Bank 2 cylinder bank.
System Too Rich (Bank 1)
ECM is subtracting more than -20% fuel trim on Bank 1 due to excess fuel or restricted airflow.
Random/Multiple Cylinder Misfire Detected
Crankshaft position sensor angular velocity fluctuations indicate combustion misfires across two or more cylinders.
Cylinder 1 Misfire Detected
Crankshaft acceleration drop isolated to Cylinder 1 firing event.
Cylinder 2 Misfire Detected
Combustion misfire isolated to Cylinder 2.
Cylinder 3 Misfire Detected
Combustion misfire isolated to Cylinder 3.
Cylinder 4 Misfire Detected
Combustion misfire isolated to Cylinder 4.
Evaporative Emission System Leak Detected (Gross Leak)
EVAP monitor failed to pull or hold vacuum in the fuel tank vapor system, indicating a large leak or missing cap.
Evaporative Emission System Leak Detected (Small Leak)
EVAP pressure decay indicates a pinhole leak (~0.040 inch / 1.0 mm equivalent) in the fuel vapor system.
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Engine coolant did not reach target operating temperature (typically 82–90 °C / 180–195 °F) within the calibrated warm-up time and airflow integral.
Exhaust Gas Recirculation (EGR) Flow Insufficient Detected
Manifold pressure or temperature change when EGR is commanded open is below expected threshold.
Mass or Volume Air Flow (MAF) A Circuit Range/Performance
Measured MAF airflow g/s deviates from calculated speed-density airflow (RPM × MAP × displacement).
Idle Air Control System RPM Higher Than Expected
Warm curb idle speed remains >150–200 RPM above target despite closed throttle.
Intake Air Temperature (IAT) Sensor 1 Circuit High
ECM detects an open circuit (>4.9 V) on the NTC thermistor IAT sensor line, interpreted as -40 °C (-40 °F).
O2 Sensor Heater Circuit (Bank 1 Sensor 1)
Current draw or resistance of the upstream air/fuel ratio sensor heater element is out of specification.
O2 Sensor Heater Circuit (Bank 1 Sensor 2)
Downstream oxygen sensor heater circuit current out of normal range.
Camshaft Position Sensor A Circuit (Bank 1 or Single Sensor)
Missing or erratic pulse train from Bank 1 intake camshaft Hall-effect or magnetoresistive sensor.
Crankshaft Position Sensor A Circuit
ECM receives no RPM signal from the crankshaft reluctor wheel during cranking or running.
Transmission Control System (MIL Request)
Informational flag set in the engine ECU indicating that the Transmission Control Module (TCM) has stored a specific gearbox fault code.
Torque Converter Clutch Circuit Performance or Stuck Off
Slip RPM across the torque converter exceeds threshold when lock-up clutch is commanded engaged at highway speed.
Turbocharger/Supercharger A Underboost Condition
Actual Manifold Absolute Pressure (MAP) remains significantly below target boost pressure for >4–5 seconds under load.
Diesel Particulate Filter (DPF) Efficiency Below Threshold (Bank 1)
Differential pressure sensor across the DPF detects abnormal pressure drop or soot pass-through.
Throttle/Pedal Position Sensor/Switch D/E Voltage Correlation
Dual redundant potentiometers or Hall tracks in the Accelerator Pedal Position (APP) sensor disagree by more than the safety threshold.
A Camshaft Position — Timing Over-Advanced or System Performance (Bank 1)
Variable Valve Timing (VVT) phaser did not return to target angle or remained over-advanced.
Crankshaft Position – Camshaft Position Correlation (Bank 1 Sensor A)
Phase angle between crankshaft reluctor and intake camshaft exceeds mechanical tolerance.
Fuel Rail/System Pressure — Too Low
High-pressure direct-injection (GDI/Common Rail) fuel rail pressure is significantly below commanded target.
Left Front Wheel Speed Sensor Supply/Circuit
ABS/ESC module detects an open circuit, short, or missing pulse signal from the front-left wheel speed sensor.
Electronic Control Unit (BCM/SRS) Internal Performance / Calibration
Body Control Module or Supplemental Restraint System controller detected an internal checksum, EEPROM, or voltage supply anomaly.
Lost Communication With ECM/PCM A
Other CAN-bus modules (TCM, ABS, Instrument Cluster) stopped receiving periodic CAN frames from the Engine Control Module.
Lost Communication With TCM (Transmission Control Module)
CAN-bus heartbeat frames from the Transmission Control Module are missing.