DPF and SCR trouble codes are stored when a diesel’s aftertreatment system, which traps soot and reduces NOx, stops meeting its targets. The diesel particulate filter, or DPF, captures soot, while selective catalytic reduction, or SCR, uses diesel exhaust fluid (DEF) to convert NOx into nitrogen and water. These codes deserve prompt attention because many … Read more
EGR system trouble codes are stored when the engine computer decides that exhaust gas recirculation is not working the way it was designed. The EGR system routes a small, metered amount of exhaust back into the intake to lower combustion temperatures and reduce oxides of nitrogen, or NOx. Because the system involves a valve, passages … Read more
Evaporative emissions codes report faults in the system that traps gasoline vapor from the fuel tank and keeps it from escaping into the air. Generic codes such as P0442 for a small leak and P0455 for a large leak are common reasons for a check engine light that comes on while the car still drives … Read more
Misfire trouble codes are stored when the engine computer detects that one or more cylinders failed to burn their air-fuel charge properly. The generic code P0300 describes a random or multiple cylinder misfire, while codes from P0301 upward identify a misfire on a specific cylinder by its number. Misfires deserve fast attention because they can … Read more
Fuel trim trouble codes appear when the engine computer has been forced to correct the air-fuel mixture further than its programming allows. Codes such as P0171, system too lean on bank 1, and P0172, system too rich on bank 1, tell you the computer has run out of room to compensate for a problem. These … Read more
Oxygen sensor circuit codes report problems the engine computer finds in the electrical side of an oxygen sensor: its heater, its signal line, or the wiring and connectors between the sensor and the module. They differ from fuel mixture codes because they question whether the sensor’s information can be trusted at all. Because the computer … Read more
Catalyst efficiency codes tell you that the engine computer no longer believes the catalytic converter is cleaning the exhaust as well as it should. The best known are P0420 and P0430, which mean catalyst system efficiency below threshold on bank 1 and bank 2, and both describe how the converter performed during a test. These … Read more
Emissions related trouble codes are the diagnostic codes OBD-II was originally created to report. They describe faults in systems that control what leaves the tailpipe and what escapes from the fuel system as vapor, and they are the codes that most directly affect the check engine light and state emissions inspections. This article explains what … Read more
Malfunction indicator lamp behavior follows rules that are far more structured than most drivers realize. The amber engine symbol does not simply react to any fault; it turns on, flashes, and turns off according to logic programmed into the powertrain control module under OBD-II requirements. Knowing that logic helps you read the lamp itself as … Read more
Check engine light codes are the stored diagnostic trouble codes that explain why the amber engine symbol on your dashboard came on. The light itself only says that a problem exists; the code, read through the diagnostic port, tells you which system or circuit the computer was watching when it found the fault. This guide … Read more