OBD-II Fault Codes

Introduction The P0140 diagnostic trouble code indicates that the downstream oxygen sensor (Bank 1, Sensor 2) is showing no voltage activity or signal output. The PCM expects a stable, somewhat flat signal from this post-catalyst sensor; complete signal loss triggers code P0140. Common Symptoms Primary Causes Step-by-Step Diagnosis & Fixes

Introduction The P0133 diagnostic trouble code indicates that the upstream oxygen sensor on Bank 1 is reacting too slowly when switching between rich and lean air-fuel mixtures. While the sensor is still functioning, its delayed response time prevents the PCM from fine-tuning fuel delivery efficiently. Common Symptoms Primary Causes Step-by-Step Diagnosis & Fixes

Introduction The P0446 diagnostic trouble code indicates an electrical or mechanical fault in the Evaporative Emission System (EVAP) vent control circuit. The vent valve allows fresh air to enter the charcoal canister during purging; when this circuit fails, the PCM cannot properly test the EVAP system for leaks. Common Symptoms Primary Causes Step-by-Step Diagnosis &

Introduction The P0304 diagnostic trouble code indicates an ignition, fuel, or mechanical failure causing a misfire specifically in Cylinder 4. The PCM monitors crankshaft acceleration drops to isolate the misfire to this specific cylinder, logging code P0304. Common Symptoms Primary Causes Step-by-Step Diagnosis & Fixes

Introduction The P0303 diagnostic trouble code indicates that the powertrain control module (PCM) has detected an engine misfire specifically in Cylinder 3. When this cylinder fails to combust its air-fuel mixture properly, it disrupts smooth crankshaft rotation, triggering a check engine light and potential catalyst damage if left unresolved. Common Symptoms Primary Causes Step-by-Step Diagnosis

Introduction The P0172 diagnostic trouble code indicates that the air-fuel mixture in Bank 1 is running too rich—meaning there is too much fuel and not enough oxygen in the exhaust stream. The powertrain control module (PCM) reduces injector pulse width to compensate, but when adjustments max out, code P0172 is stored, leading to poor fuel

Introduction The P0400 diagnostic trouble code indicates a malfunction in the Exhaust Gas Recirculation (EGR) system flow. The EGR system routes a measured amount of exhaust gases back into the engine intake manifold to lower combustion temperatures and reduce harmful nitrogen oxide ($NO_x$) emissions. When flow is absent, excessive, or erratic, the powertrain control module

Introduction The P0302 diagnostic trouble code indicates that the powertrain control module (PCM) has detected a specific misfire in Cylinder 2. Similar to P0301, this code pinpoints ignition, fuel delivery, or mechanical compression failure isolated to the second cylinder, causing irregular engine rotation and emissions spikes. Common Symptoms Primary Causes Step-by-Step Diagnosis & Fixes

Introduction The P0301 diagnostic trouble code indicates that the powertrain control module (PCM) has detected a specific misfire in Cylinder 1. When Cylinder 1 fails to ignite its air-fuel mixture properly, the crankshaft slows down momentarily during its power stroke. Unlike random misfire codes (P0300), P0301 singles out Cylinder 1, narrowing down ignition, fuel, or

Introduction The P0128 diagnostic trouble code indicates that the engine coolant temperature (ECT) has failed to reach normal operating temperature within a specified period of time. The powertrain control module (PCM) monitors engine warm-up time to optimize fuel injection and emissions control. If the engine remains too cold for too long, the computer logs code