The check engine light is one of the most important indicators in your vehicle. When it illuminates, it means there is an issue that needs addressing to keep your car running smoothly and to pass roadworthy inspections. However, making sense of engine light warnings can be daunting, as there are many possible causes. This article will break down the 4 most common check engine light codes and provide tips to help diagnose the underlying issue.
P0171 or P0174 System Too Lean/Rich Codes
One of the most frequent check engine light codes is P0171 or P0174, which indicates that the engine is running too lean (P0171) or too rich (P0174). This means that the fuel-air mixture going into the engine is not optimal. A lean mixture has too little fuel, while a rich mixture has too much fuel for the amount of air. Both conditions can cause drivability issues and reduce fuel efficiency.
To diagnose, start by checking for vacuum leaks which can allow unmetered air into the intake and upset the fuel-air ratio. Perform a smoke test on the intake to check for leaks. Also inspect fuel injectors, throttle body, and mass air flow sensor for problems. You need to check Abnormal readings using a check engine light diagnosis that can detect faulty mass air flow sensors, oxygen sensors, or fuel delivery issues requiring further diagnosis. Fixing the root cause should turn off the code.
P0300 Random/Multiple Cylinder Misfire Detected
The P0300 misfire code means that the engine control module has detected an issue causing one or more cylinders to misfire intermittently. Misfires waste fuel and cause higher emissions, preventing vehicles from passing mandatory roadworthy inspections.
To pinpoint which cylinder(s) are misfiring, a scan tool that can display individual cylinder misfire data is needed. Once the problem cylinder is identified, check the spark plugs, ignition coils, fuel injectors, and combustion chambers for that cylinder. A compression/leak-down test will reveal issues like low compression or blown head gaskets. Fouled spark plugs are a common cause and can usually be resolved with a plug replacement.
P0420 Catalytic Converter Efficiency below Threshold
The P0420 diagnostic trouble code indicates that the vehicle’s emission control system has detected a problem with the catalytic converter. This after-treatment device plays a key role in reducing vehicle emissions to legally mandated levels, and a faulty catalytic converter will prevent a vehicle from passing an emission test required during roadworthy inspections.
Typical causes could be a degraded catalytic converter due to high mileage, problems further upstream like oxygen sensor issues, fuel delivery problems, or ignition problems causing raw or improperly treated exhaust gasses to enter the catalytic converter. A scan tool can retrieve further details from stored diagnostic trouble codes to help narrow down the faulty component. Replacing a clogged or deteriorated catalytic converter usually resolves the problem.
P0135 & P0141 Oxygen Sensor Circuit Malfunctions
These codes refer to malfunctions detected in the front and rear oxygen sensor (O2 sensor) circuits. As the O2 sensors are an essential component in the closed-loop fuel control system, a faulty sensor will prevent the engine control module from maintaining the ideal fuel mixture. This hurts fuel economy and emission levels.
Common issues include a damaged or aged O2 sensor reaching the end of its lifespan, or wiring harness faults. A scan tool can reveal which sensor circuit has malfunctioned. Inspecting the sensor, its wiring, and connectors is needed. If the component checks out fine, a wiring issue between the sensor and engine control module is indicated.
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