OBD-II Codes — SAE J2012 / ISO 15031 Diagnostic Lookup
Decode Check Engine and ABS/ESP fault codes with verified symptoms, root causes, and multimeter/scanner diagnostic steps.
| Standard Reference | SAE J2012 / ISO 15031-6 Diagnostic Trouble Codes (DTC) |
|---|---|
| Covered Systems | P0xxx Powertrain, C0xxx ABS/Chassis, U0xxx CAN Network |
| Codes in Reference | 32 High-Frequency Trouble Codes with Root Causes & Diagnostic Steps |
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)powertrain · Severity: medium · Oxygen sensor 2 (downstream) waveform mirrors upstream O2 sensor, indicating reduced oxygen storage capacity in the three-way catalytic converter.P0171 — System Too Lean (Bank 1)powertrain · Severity: medium · 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.P0174 — System Too Lean (Bank 2)powertrain · Severity: medium · Excessive positive fuel trim correction on Bank 2 cylinder bank.P0172 — System Too Rich (Bank 1)powertrain · Severity: medium · ECM is subtracting more than -20% fuel trim on Bank 1 due to excess fuel or restricted airflow.P0300 — Random/Multiple Cylinder Misfire Detectedpowertrain · Severity: high · Crankshaft position sensor angular velocity fluctuations indicate combustion misfires across two or more cylinders.P0301 — Cylinder 1 Misfire Detectedpowertrain · Severity: high · Crankshaft acceleration drop isolated to Cylinder 1 firing event.P0302 — Cylinder 2 Misfire Detectedpowertrain · Severity: high · Combustion misfire isolated to Cylinder 2.P0303 — Cylinder 3 Misfire Detectedpowertrain · Severity: high · Combustion misfire isolated to Cylinder 3.P0304 — Cylinder 4 Misfire Detectedpowertrain · Severity: high · Combustion misfire isolated to Cylinder 4.P0455 — Evaporative Emission System Leak Detected (Gross Leak)powertrain · Severity: low · EVAP monitor failed to pull or hold vacuum in the fuel tank vapor system, indicating a large leak or missing cap.P0442 — Evaporative Emission System Leak Detected (Small Leak)powertrain · Severity: low · EVAP pressure decay indicates a pinhole leak (~0.040 inch / 1.0 mm equivalent) in the fuel vapor system.P0128 — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)powertrain · Severity: low · Engine coolant did not reach target operating temperature (typically 82–90 °C / 180–195 °F) within the calibrated warm-up time and airflow integral.P0401 — Exhaust Gas Recirculation (EGR) Flow Insufficient Detectedpowertrain · Severity: medium · Manifold pressure or temperature change when EGR is commanded open is below expected threshold.P0101 — Mass or Volume Air Flow (MAF) A Circuit Range/Performancepowertrain · Severity: medium · Measured MAF airflow g/s deviates from calculated speed-density airflow (RPM × MAP × displacement).P0507 — Idle Air Control System RPM Higher Than Expectedpowertrain · Severity: low · Warm curb idle speed remains >150–200 RPM above target despite closed throttle.P0113 — Intake Air Temperature (IAT) Sensor 1 Circuit Highpowertrain · Severity: low · ECM detects an open circuit (>4.9 V) on the NTC thermistor IAT sensor line, interpreted as -40 °C (-40 °F).P0135 — O2 Sensor Heater Circuit (Bank 1 Sensor 1)powertrain · Severity: medium · Current draw or resistance of the upstream air/fuel ratio sensor heater element is out of specification.P0141 — O2 Sensor Heater Circuit (Bank 1 Sensor 2)powertrain · Severity: low · Downstream oxygen sensor heater circuit current out of normal range.P0340 — Camshaft Position Sensor A Circuit (Bank 1 or Single Sensor)powertrain · Severity: high · Missing or erratic pulse train from Bank 1 intake camshaft Hall-effect or magnetoresistive sensor.P0335 — Crankshaft Position Sensor A Circuitpowertrain · Severity: high · ECM receives no RPM signal from the crankshaft reluctor wheel during cranking or running.P0700 — Transmission Control System (MIL Request)powertrain · Severity: medium · Informational flag set in the engine ECU indicating that the Transmission Control Module (TCM) has stored a specific gearbox fault code.P0741 — Torque Converter Clutch Circuit Performance or Stuck Offpowertrain · Severity: medium · Slip RPM across the torque converter exceeds threshold when lock-up clutch is commanded engaged at highway speed.P0299 — Turbocharger/Supercharger A Underboost Conditionpowertrain · Severity: medium · Actual Manifold Absolute Pressure (MAP) remains significantly below target boost pressure for >4–5 seconds under load.P2002 — Diesel Particulate Filter (DPF) Efficiency Below Threshold (Bank 1)powertrain · Severity: medium · Differential pressure sensor across the DPF detects abnormal pressure drop or soot pass-through.P2138 — Throttle/Pedal Position Sensor/Switch D/E Voltage Correlationpowertrain · Severity: high · Dual redundant potentiometers or Hall tracks in the Accelerator Pedal Position (APP) sensor disagree by more than the safety threshold.P0011 — A Camshaft Position — Timing Over-Advanced or System Performance (Bank 1)powertrain · Severity: medium · Variable Valve Timing (VVT) phaser did not return to target angle or remained over-advanced.P0016 — Crankshaft Position – Camshaft Position Correlation (Bank 1 Sensor A)powertrain · Severity: high · Phase angle between crankshaft reluctor and intake camshaft exceeds mechanical tolerance.P0087 — Fuel Rail/System Pressure — Too Lowpowertrain · Severity: high · High-pressure direct-injection (GDI/Common Rail) fuel rail pressure is significantly below commanded target.C0035 — Left Front Wheel Speed Sensor Supply/Circuitchassis · Severity: medium · ABS/ESC module detects an open circuit, short, or missing pulse signal from the front-left wheel speed sensor.B1000 — Electronic Control Unit (BCM/SRS) Internal Performance / Calibrationbody · Severity: medium · Body Control Module or Supplemental Restraint System controller detected an internal checksum, EEPROM, or voltage supply anomaly.U0100 — Lost Communication With ECM/PCM Anetwork · Severity: high · Other CAN-bus modules (TCM, ABS, Instrument Cluster) stopped receiving periodic CAN frames from the Engine Control Module.U0101 — Lost Communication With TCM (Transmission Control Module)network · Severity: high · CAN-bus heartbeat frames from the Transmission Control Module are missing.