Check Engine Light Codes Explained: P0420, P0300, P0455 and Other Common Codes

Read the trouble code. Every car and light truck sold in the U.S. since the 1996 model year has an OBD-II port under the dashboard near the steering column. A basic code reader or Bluetooth adapter costs about $20 to $30, and many auto parts stores will read the code for free. Codes starting with P0 are standard across all brands; codes starting with P1 are specific to your manufacturer. The code tells you which system has a problem, not always which part to replace.

No, not far. A flashing check engine light means a severe misfire: unburned fuel is going into the exhaust and can overheat and destroy the catalytic converter, which is an expensive repair. Slow down, avoid hard acceleration, and get it checked right away; if the car shakes badly or loses power, pull over and have it towed. A steady light is less urgent, but still get the code read soon.

These are misfire codes. P0300 means random misfires; P0301 through P0308 point to a specific cylinder, such as P0302 for cylinder 2. The most common causes are worn spark plugs and a failing ignition coil. A simple test is to swap that cylinder’s coil with a neighboring one, clear the code and drive: if the misfire moves to the other cylinder, the coil is bad. Vacuum leaks and clogged fuel injectors are the next things to check.

P0420 means the catalytic converter isn’t cleaning the exhaust as well as it should, as judged by the oxygen sensors before and after it. Before replacing the converter, which is one of the most expensive check engine repairs, rule out a failing downstream oxygen sensor, an exhaust leak ahead of the sensor, and misfires that may have damaged it. If the converter really has failed, check your warranty: federal emissions warranty covers catalytic converters for 8 years or 80,000 miles on most vehicles.

Yes. Codes like P0455 (large leak), P0456 (very small leak) and P0442 (small leak) come from the evaporative emission system, which traps fuel vapor, and a loose, cracked or worn gas cap is the most common cause. Tighten the cap until it clicks, or replace it if the seal is cracked. The light won’t go off right away; it can take several days of normal driving. If it comes back, a shop can smoke-test the system to find the leak.

The engine is getting too much air or too little fuel. P0171 is one side of the engine, P0174 the other; both together usually point to something shared. The most common causes are a vacuum leak from a cracked hose or intake gasket, a dirty mass air flow sensor (cleaning it with spray made for MAF sensors often fixes it), or weak fuel pressure from a clogged filter or tired fuel pump.

P0128 means the engine is taking too long to warm up, almost always because the thermostat is stuck open. You may notice the temperature gauge reading lower than usual, weak heat from the heater in cold weather, and worse fuel mileage. Replacing the thermostat is a moderate DIY job on many cars. Low coolant can also cause it, so check the level when the engine is cold.

Clearing the code turns the light off, but it comes back if the problem is still there, and clearing also resets the car’s readiness monitors. In Arizona’s emissions-testing areas around Phoenix and Tucson, a car with the check engine light on fails, and a car whose code was just cleared usually can’t be tested until it has been driven enough, often several days of mixed driving, for the monitors to show ready. Fix the cause first, then drive normally before testing.