A handful of codes dominates every check-engine search log. P0420 alone out-searches most entire car models. Here is the leaderboard with what each code actually means, the usual culprit, and the honest national fix-cost band — plus the two codes you should never sit on.
| Code | Meaning | Usual Culprit | Typical Fix Cost |
|---|---|---|---|
| P0420 | Catalyst efficiency below threshold (bank 1) | Aging converter, or lazy downstream O2 sensor | $150–$400 (O2) · $1,300–$3,000+ (cat) |
| P0171 | System too lean (bank 1) | Vacuum leak, dirty MAF, weak fuel pump | $100–$500 |
| P0300 | Random misfire | Plugs, coils, occasionally injector or compression | $150–$600 |
| P0442 | EVAP small leak | Cracked hose, purge or vent valve | $100–$450 |
| P0455 | EVAP large leak | Gas cap loose or bad | $15–$50 (cap) |
| P0430 | Catalyst efficiency (bank 2) | Same as P0420, second bank | $1,300–$3,000+ |
| P0128 | Thermostat below regulating temp | Thermostat stuck open | $250–$550 |
| P0135 | O2 heater circuit (bank 1 sensor 1) | Dead O2 sensor heater | $150–$400 |
| P0401 | EGR insufficient flow | Carbon-clogged EGR valve or ports | $150–$700 |
| P0299 | Turbo underboost | Boost leak or wastegate; turbo worst case | $100–$3,000 |
| P0301–P0306 | Specific cylinder misfire | Plug or coil on that cylinder | $100–$500 |
| P0507 | Idle RPM higher than expected | Dirty throttle body, vacuum leak | $100–$500 |
Costs are typical US independent-shop ranges, parts plus labor, 2025–26. Dealer rates run 20–40% higher, and hard-to-reach sensors on transverse V6s can double the labor on a job that looks trivial.
Type any generic P-code and get the meaning, likely causes in order, and the fix-cost band.
OBD2 Code Lookup →The EVAP family (P0440, P0442, P0455, P0456) is emissions-only. The vapor recovery system keeps fuel fumes out of the air; a leak affects nothing you can feel. Drive it, fix it before the smog check, and always spend the first five minutes checking that the gas cap clicks and its seal isn't cracked. That $15 part resolves a surprising share of P0455s.
A steady P0420 or P0430 on an older car is a judgment call. The converter is a $1,300-plus part on a car that might be worth $6,000, and the car runs fine without it doing its job. The catch: you cannot legally remove it, you will fail emissions testing where it is required, and the code hides real problems. Fix any misfire or lean code first, because those are what kill converters — replacing a converter while a misfire persists means buying two.
Misfires. A flashing check engine light is the computer begging you to stop: raw fuel is entering the exhaust and burning inside the catalytic converter, which overheats and destroys it. People turn a $200 plug-and-coil job into a $2,000 converter-plus-plugs job by driving on a flashing light for a week. The same goes for lean codes under load (P0171/P0174 that appear while towing or climbing), which run combustion hot, and any overtemperature code.
The format carries information before you look anything up. First character: P powertrain, B body, C chassis, U network. Second digit: 0 means generic (SAE standardized across every make), 1 means manufacturer-specific — a P1xxx on a Honda can mean something different than on a Ford. Third digit is the subsystem: 1 fuel and air metering, 3 ignition and misfires, 4 auxiliary emissions, 5 idle and speed control. Last two digits identify the fault itself.
So P0304 parses as: generic powertrain, ignition/misfire, cylinder 4. That tells you the test before you spend anything: move the cylinder 4 coil to cylinder 2, clear the code, and see if the misfire follows the coil (bad coil) or stays with the hole (plug, injector, or compression). Twenty minutes, zero parts.
A Bluetooth OBD2 adapter costs $20–$40 and pairs with a free phone app to read and clear generic codes. That turns every dashboard light from a mystery into a sentence you can search. Parts stores read codes free too, but having your own means you can check whether a fix worked, watch misfire counters in real time, and catch pending codes before the light comes on. Once you know the code, the repair cost calculator prices the job, and if it points to the converter, the catalytic converter cost calculator breaks down parts by source and the theft-scenario math.
P0420, catalyst system efficiency below threshold on bank 1. It is the most searched OBD2 code by a wide margin because it is ambiguous: the converter might be done, or a $150-$400 downstream O2 sensor might be lying about it. Diagnose before replacing parts.
EVAP codes (P0440 family) are emissions-only: the car drives fine and the only casualty is the smog check. A steady P0420 on an old car is a judgment call. Never ignore misfire codes (P0300-P0308) with a flashing light, lean codes under load, or an overtemperature code; those destroy engines and converters.
If the downstream O2 sensor is the problem, $150 to $400 installed. If the catalytic converter itself is degraded, $1,300 to $3,000 or more for a compliant direct-fit unit, higher with OEM parts, CARB-compliant units in California and New York, or theft damage. Price your exact case before deciding whether an older car is worth it.
Not immediately, but do not leave it for a year. System-too-lean means the engine is running with more air than the computer can fuel for, usually a vacuum leak, dirty mass airflow sensor, or weak fuel delivery. It costs fuel economy and power, and sustained lean running overheats combustion and kills converters. Most fixes land between $100 and $500.
Because clearing a code does not fix the fault. The computer re-runs its monitor, re-detects the problem, and re-sets the light, usually within a few drive cycles. A code that returns quickly is a confirmed fault; that is your signal to diagnose it instead of clearing it again.