A trouble code is four characters, a letter and three numbers, and it tells you almost nothing on its own. P0420 sounds identical in severity to P0300 if you don't know what either one means, but one might cost you a gas cap and the other might cost you a new catalytic converter. Here are the five codes that show up most often on OBD2 scanners across every make and model, what triggers each one, and what kind of repair bill you're realistically looking at.
P0300: Random/Multiple Cylinder Misfire Detected
P0300 means your engine's computer detected a misfire and can't pin it to one specific cylinder. If it could isolate the cylinder, you'd see P0301 through P0308 instead, with the last digit matching the cylinder number. A random misfire across multiple cylinders points to something feeding the whole engine rather than one bad component in one spot.
The usual causes, roughly in order of how often they show up: worn spark plugs that have gone past their service interval, a fuel delivery issue like a weak fuel pump or clogged injectors, a vacuum leak pulling unmetered air into the intake, or a bad ignition coil pack on engines that share one coil across multiple cylinders.
Spark plugs are the cheapest fix on this list, often under $100 for parts and labor on most four-cylinder engines. A failing fuel pump runs considerably higher, frequently $400 to $800 depending on the vehicle. The real risk with P0300 isn't the code itself. It's what happens if you keep driving on an active misfire: unburned fuel reaches the catalytic converter and can destroy it, turning a $100 spark plug job into a $1,500 exhaust repair. If this code shows up alongside a flashing check engine light, stop driving on it and get it checked.
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 has a reputation as the most misdiagnosed code on any scanner, and that reputation is earned. The code technically means your catalytic converter isn't cleaning exhaust gases as efficiently as the computer expects, based on comparing the oxygen sensor readings before and after the converter. What it doesn't tell you is why.
A dying catalytic converter is one possible cause, and replacing one runs anywhere from $300 for an aftermarket unit on an older sedan to $2,500 or more for an OEM part on certain trucks and imports. But a failing oxygen sensor downstream of the converter throws the exact same code and costs a fraction to fix, usually $150 to $300 installed. An exhaust leak upstream of the sensor can also trigger P0420 by letting extra oxygen skew the reading. Even an unrelated engine misfire can damage a converter over time and eventually show up as this code once the damage is done.
Before replacing a catalytic converter over a P0420 code, check the oxygen sensors first and inspect the exhaust system for leaks. Skipping straight to converter replacement is the single most common way people overspend on this particular code.
P0171: System Too Lean (Bank 1)
P0171 means your engine is getting too much air relative to the fuel it's burning, based on the oxygen sensor and fuel trim data the computer is tracking. Engines run on a fairly narrow air-to-fuel ratio, and drifting lean forces the computer to add more fuel than the design ever anticipated, which shows up as a high positive fuel trim number.
Vacuum leaks cause this more than anything else on the list. A cracked intake hose, a bad gasket, or a loose connection anywhere in the intake system lets unmetered air into the engine, and that extra air throws off the whole ratio. A dirty or failing mass airflow sensor is the second most common cause, since it directly controls how much fuel the computer thinks it needs to add. A weak fuel pump or clogged fuel filter can also produce this code by starving the engine of fuel rather than flooding it with air, even though the end result looks the same to the computer.
Repair costs vary enormously here because the cause varies enormously. A vacuum leak from a cracked hose might cost $20 in parts. A mass airflow sensor replacement runs $150 to $350 depending on the vehicle. Don't guess on this one. Pull the live fuel trim data with a decent scanner and watch how the numbers change with the engine running versus idling, since that behavior narrows the cause considerably before you buy a single part.
P0455: Evaporative Emission System Leak Detected (Large Leak)
This is the code that makes people panic over nothing. P0455 means the evaporative emissions system, the sealed system that captures fuel vapor instead of releasing it into the air, has detected a large leak. In the overwhelming majority of cases, that leak is a gas cap that wasn't tightened properly or has a worn seal.
Before doing anything else, check the gas cap. Make sure it clicks when you tighten it, and look at the rubber seal for cracks or debris. If the cap looks fine, drive for a day or two and see if the code clears itself, since some systems need a full drive cycle to confirm the leak is gone after a fix.
If a new gas cap doesn't resolve it, the next suspects are a cracked EVAP hose, a bad purge valve, or a failed vent valve, none of which cost much to replace individually, usually $50 to $200 depending on the part. This is one of the lowest-stakes codes on this entire list. It won't damage your engine and it's rarely expensive to fix. It exists mainly because emissions regulations require the system to be monitored.
P0128: Coolant Thermostat Below Regulating Temperature
P0128 shows up when your engine takes too long to reach normal operating temperature, which almost always points to a stuck-open thermostat. The thermostat's job is to block coolant flow to the radiator until the engine warms up, and when it fails open, coolant keeps circulating and pulls heat away before the engine ever gets to temperature.
A stuck-open thermostat isn't an emergency, but it isn't harmless either. Running cold longer than intended increases engine wear over time and hurts fuel economy, since the computer runs a richer fuel mixture during warm-up. Thermostats are inexpensive parts, often $15 to $40, but labor varies a lot by vehicle depending on how buried it is in the engine bay. Total repair cost typically lands between $150 and $300.
A less common cause is a coolant temperature sensor reading incorrectly rather than the thermostat failing at all, which live data can confirm fast: watch the coolant temp reading during a cold start and compare how long it takes to climb against a known-good baseline for your engine.
The Pattern Across All Five
Every code on this list means roughly the same thing at the mechanical level: a sensor sent the computer a number outside its expected range, and the computer flagged it. None of these codes name the failed part directly, no matter how specific the wording sounds. P0420 says "catalyst efficiency," not "catalytic converter is dead." P0171 says "system too lean," not "vacuum leak at hose X."
A scanner that shows live data instead of only the stored code turns each of these from a guess into a confirmed diagnosis, and that's the difference between replacing the part that's failing and replacing three parts before finding the right one.