OBD-II Trouble Code Directory

Catalyst System Efficiency Below Threshold

Code P0420 indicates that the catalytic converter is not operating at peak efficiency. The downstream oxygen sensor detects that the converter is not reducing exhaust emissions as effectively as expected. This is one of the most common OBD-II codes.

Random/Multiple Cylinder Misfire Detected

Code P0300 indicates that the engine control module has detected misfires occurring in multiple cylinders at random. Unlike cylinder-specific misfire codes, P0300 points to a problem that affects the entire engine rather than a single cylinder, such as a vacuum leak, fuel delivery issue, or ignition system fault.

System Too Lean (Bank 1)

Code P0171 indicates that the engine control module has detected a lean air-fuel mixture on bank 1 of the engine. This means there is too much air or not enough fuel entering the combustion chambers. The ECM tries to compensate by adding fuel, but when the correction exceeds a threshold, the code is set.

Coolant Thermostat Below Thermostat Regulating Temperature

Code P0128 indicates that the engine coolant temperature has not reached the expected operating temperature within a specified time after starting. This typically means the thermostat is stuck open, allowing coolant to flow continuously through the radiator and preventing the engine from warming up properly.

Evaporative Emission Control System Leak Detected (Small Leak)

Code P0442 indicates that the EVAP system has detected a small vapor leak. The EVAP system prevents fuel vapors from escaping into the atmosphere. The ECM tests this system by pressurizing it and monitoring for pressure drops. A small leak triggers P0442, which is often caused by something as simple as a loose gas cap.

Evaporative Emission Control System Leak Detected (Gross Leak)

Code P0455 indicates that the EVAP system has detected a large vapor leak. This is a more significant leak than P0442 and usually points to a missing gas cap, a disconnected EVAP hose, or a major component failure in the evaporative emission system. The gross leak designation means the system cannot maintain any meaningful pressure.

Catalyst System Efficiency Below Threshold (Bank 2)

Code P0430 is the bank 2 equivalent of P0420. It indicates that the catalytic converter on the second bank of a V-engine is not operating efficiently. The downstream oxygen sensor detects that exhaust gases are not being properly converted, meaning the catalytic converter is deteriorating or another issue is affecting its performance.

Evaporative Emission Control System Malfunction

Code P0440 is a general EVAP system malfunction code indicating that the system is not functioning as designed. Unlike the more specific P0442 or P0455 codes, P0440 does not specify the size of a leak. It may indicate a leak, a faulty sensor, or an issue with a solenoid valve in the EVAP system.

Exhaust Gas Recirculation Flow Insufficient

Code P0401 indicates that the EGR (Exhaust Gas Recirculation) system is not flowing enough exhaust gas back into the intake manifold. The EGR system reduces combustion temperatures and NOx emissions by recirculating a portion of exhaust gas. When the flow is insufficient, the ECM detects higher-than-expected combustion temperatures or uses a DPFE sensor to measure the shortfall.

Evaporative Emission Control System Vent Control Circuit Malfunction

Code P0446 indicates a problem with the EVAP system vent control circuit. The vent valve allows fresh air into the charcoal canister during purge cycles and seals the system during leak tests. When the ECM detects an electrical or mechanical fault in the vent valve circuit, P0446 is set.

Cylinder 1 Misfire Detected

Code P0301 indicates that the engine control module has detected a misfire specifically in cylinder 1. A misfire occurs when the air-fuel mixture in the cylinder fails to ignite properly. The ECM detects this by monitoring crankshaft speed variations. Cylinder-specific misfires are generally easier to diagnose than random misfires because the problem is isolated to one cylinder.

Cylinder 2 Misfire Detected

Code P0302 indicates that the engine control module has detected a misfire in cylinder 2. The ECM monitors crankshaft rotational velocity and identifies that cylinder 2 is not contributing its expected power. This could be due to ignition, fuel delivery, or mechanical issues specific to that cylinder.

Driver Frontal Stage 1 Deployment Control

Code B0001 indicates a malfunction in the driver-side frontal airbag stage 1 deployment circuit. The airbag control module has detected an open circuit, high resistance, or short in the wiring leading to the driver airbag inflator. This is a safety-critical fault because the driver airbag may not deploy in a collision.

Driver Frontal Stage 2 Deployment Control

Code B0002 indicates a fault in the driver-side frontal airbag stage 2 deployment circuit. Stage 2 is a secondary inflator that provides additional cushioning during severe frontal collisions. A failure here means the airbag may partially deploy but not provide full protection in a high-severity impact.

Passenger Frontal Stage 1 Deployment Control

Code B0015 indicates a malfunction in the passenger-side frontal airbag stage 1 deployment circuit. The SRS module has detected an electrical fault such as an open or short circuit in the passenger airbag inflator wiring. The passenger airbag may fail to deploy in a frontal collision.

Passenger Frontal Stage 2 Deployment Control

Code B0016 indicates a fault in the passenger-side frontal airbag stage 2 deployment circuit. Similar to B0002 for the driver side, this code means the secondary inflator on the passenger airbag has an electrical problem. Full airbag protection during severe impacts is compromised.

Driver Side Airbag Deployment Loop Resistance High

Code B0028 indicates that the resistance in the driver-side airbag deployment loop is higher than the expected range. High resistance in the airbag circuit can prevent the inflator from firing reliably. This is often caused by a deteriorating clockspring or corroded connections in the steering column.

Rear Body Right Turn Signal Circuit Malfunction

Code B0050 indicates that the body control module has detected a fault in the right rear turn signal circuit. This could be an open circuit from a burned-out bulb, excessive resistance in the wiring, or a short to ground. The BCM monitors current draw to detect bulb failures and circuit problems.

Rear Body Left Turn Signal Circuit Malfunction

Code B0051 indicates a fault in the left rear turn signal circuit as detected by the body control module. The BCM monitors the circuit current and has detected an abnormality such as an open circuit, short, or excessive resistance. This is the left-side counterpart to B0050.

Passenger Seat Position Sensor Circuit Malfunction

Code B0081 indicates that the SRS or body control module has detected a fault in the passenger seat position sensor circuit. This sensor determines how far forward or rearward the passenger seat is positioned and adjusts airbag deployment force accordingly. A faulty sensor means the airbag system cannot optimize deployment for the passenger.

Passenger Presence System Malfunction

Code B0092 indicates that the passenger occupant detection system has malfunctioned. This system uses a sensor in the passenger seat to determine whether a passenger is present and their approximate weight. The SRS module uses this data to decide whether to enable or suppress the passenger airbag deployment.

Exterior Lighting Headlamp Output Circuit Malfunction

Code B0100 indicates that the body control module has detected a fault in the headlamp output circuit. The BCM monitors current flow to the headlamps and has detected an abnormal condition such as an open circuit, short to ground, or overcurrent. This can affect one or both headlamps depending on the specific circuit affected.

Left Front Wheel Speed Sensor Circuit Malfunction

Code C0035 indicates that the ABS control module has detected a fault in the left front wheel speed sensor circuit. The sensor generates a signal proportional to wheel speed that the ABS, traction control, and stability control systems rely on. A faulty signal means these safety systems cannot function correctly for that wheel.

Right Front Wheel Speed Sensor Circuit Malfunction

Code C0040 indicates a fault in the right front wheel speed sensor circuit as detected by the ABS module. This sensor provides critical speed data for ABS, traction control, and electronic stability control. The code is set when the sensor signal is missing, erratic, or outside the expected range.

Left Rear Wheel Speed Sensor Circuit Malfunction

Code C0050 indicates that the ABS control module has detected a problem with the left rear wheel speed sensor circuit. Rear wheel speed sensors are particularly prone to damage from road debris, water, and salt exposure. Without a valid signal, ABS and stability control cannot properly modulate that wheel.

Right Rear Wheel Speed Sensor Circuit Malfunction

Code C0060 indicates a fault in the right rear wheel speed sensor circuit. The ABS module has lost a valid speed signal from the right rear wheel. This sensor is essential for ABS, traction control, and stability control operation on the right rear wheel and is commonly damaged by road conditions.

Pump Motor Circuit Malfunction

Code C0110 indicates that the ABS control module has detected a malfunction in the ABS hydraulic pump motor circuit. The pump motor is responsible for generating hydraulic pressure to modulate brake force during ABS events. A failed pump means ABS cannot reduce wheel lockup during emergency braking.

ABS/TCS Brake Switch Circuit Malfunction

Code C0200 indicates that the ABS and traction control system has detected a problem with the brake switch circuit. The brake switch tells the ABS module when the driver is pressing the brake pedal. Without this input, the ABS and traction control systems cannot determine driver intent and may be disabled.

Rear Speed Sensor Circuit Malfunction

Code C0300 indicates a general rear speed sensor circuit fault detected by the ABS module. On some vehicles, particularly those with a rear-wheel ABS system, a single sensor on the rear axle or differential provides speed data. This code means that sensor circuit has an electrical problem preventing accurate speed measurement.

High Speed CAN Communication Bus Fault

Code U0001 indicates a malfunction on the high-speed Controller Area Network (CAN) bus. The CAN bus is the primary communication backbone that connects all major control modules in the vehicle. A fault here means multiple modules may lose the ability to communicate, potentially disabling numerous vehicle systems simultaneously.

Control Module Communication Bus Off

Code U0073 indicates that a control module has detected that the communication bus has gone offline. This means the CAN bus has experienced enough errors that the module has entered a bus-off state, effectively disconnecting from all other modules. This is a protective mechanism but results in loss of module communication.

Lost Communication with ECM/PCM

Code U0100 indicates that one or more control modules have lost communication with the Engine Control Module (ECM) or Powertrain Control Module (PCM). The ECM/PCM is the primary engine management computer, and other modules rely on data from it. Loss of communication typically results in a no-start or stalling condition.

Lost Communication with TCM

Code U0101 indicates that a control module has lost communication with the Transmission Control Module (TCM). The TCM manages automatic transmission shifting and relies on CAN bus data from the ECM and other modules. Loss of communication may cause the transmission to default to a limp-mode gear.

Lost Communication with ABS Control Module

Code U0121 indicates that a control module has lost CAN bus communication with the Anti-lock Brake System (ABS) control module. The ABS module shares wheel speed data and brake status with other systems including traction control, stability control, and the instrument cluster. Loss of communication disables these integrated safety features.

Lost Communication with Body Control Module

Code U0140 indicates that one or more control modules have lost CAN bus communication with the Body Control Module (BCM). The BCM controls many convenience and safety features including lighting, door locks, windows, and security. Loss of BCM communication can cause widespread electrical symptoms throughout the vehicle.

Lost Communication with Restraints Control Module (SRS)

Code U0151 indicates that the vehicle's CAN bus has lost communication with the Supplemental Restraint System (SRS) control module, also known as the airbag control module. This module manages all airbag deployments, seatbelt pretensioners, and crash sensors. Without communication, the entire airbag system is disabled.

Lost Communication with Instrument Panel Cluster (IPC)

Code U0155 indicates that other control modules have lost CAN bus communication with the Instrument Panel Cluster (IPC). The IPC receives data from many modules to display speed, RPM, fuel level, temperature, and warning lights. When communication is lost, the driver loses visibility into critical vehicle information.