The 10 Most Common Check Engine Light Codes (What They Mean and What They Cost)

Pull the codes from enough cars and the same handful shows up again and again. Emissions systems, misfires, lean mixtures, evap leaks — the greatest hits are remarkably consistent across brands, because every OBD-II car watches the same systems in the same ways. Below are the ten codes drivers run into most, roughly in the order you are likely to meet them, with what each one means in plain English, the cause that most often turns out to be behind it in the real world, and an honest sense of cost.

Two things before the list. First, cost brackets here are deliberately broad and in words, not fake-precise dollar figures — the closing section explains why the same code can be a cheap fix on one car and a painful one on another. Second, a code identifies a system, not a part; if that idea is new to you, read what a check engine light really means first. And if your code is not on this list, our OBD code lookup covers all of them.

A note on the rankings: these are not laboratory-precise national statistics, because no honest source has those. They reflect what actually comes up over and over in shops, forums, and diagnostic data — the codes a scanner is most likely to hand you on any given day. Your own odds shift with what you drive and how old it is. Older, higher-mileage cars skew toward the catalyst and misfire codes at the top of the list; nearly anything with a filler neck can produce the evap codes in the middle of it. Treat the order as a rough map of the territory, not a prediction for your driveway.

One code on this list comes with a real deadline: a misfire with a flashing check-engine light is actively damaging your catalytic converter. If the light is flashing, ease off and get it looked at now — everything else here can wait for a calm read.

1. P0420 — Catalyst efficiency below threshold

In one sentence: the catalytic converter is not cleaning the exhaust as well as the computer expects. The reflexive fix is a new converter, and that is often wrong — the most common real-world story is an aging downstream oxygen sensor skewing the comparison, or an upstream problem (a lingering misfire, a lean condition) degrading the converter's performance without killing it. Genuinely dead converters usually belong to high-mileage cars or ones that ran with an ignored misfire. Cost runs from modest — an oxygen sensor and an hour of labor — to one of the most expensive routine repairs on a car if the converter truly needs replacing, especially with a factory part. Diagnose before you buy: the full breakdown is at P0420.

2. P0300 — Random or multiple misfires

In one sentence: cylinders are failing to fire properly, and not just one of them — the engine is stumbling across the board. The most common real cause is simple neglect of ignition parts: worn spark plugs, tired coils, old wires. But because P0300 is scattered rather than isolated, it can also point at something all cylinders share — a vacuum leak, low fuel pressure, or a clogged filter. You will usually feel it as a rough idle or hesitation. Cost is friendly if it is plugs, moderate if coils join the party, and climbs if the cause is fuel delivery or something mechanical. Ignore it and the converter pays the price, which is how a cheap code becomes an expensive one. Details at P0300.

3. P0171 — System too lean (bank 1)

In one sentence: the engine is getting more air than the fuel it measures for, so the computer is constantly adding fuel to compensate. The most common real cause is unmetered air — a cracked or loose vacuum hose, a leaking intake gasket — with a dirty mass-airflow sensor a close second, since a lazy sensor under-reports the air coming in. Weak fuel delivery (a tired pump, a clogged filter) is the less common but pricier suspect. Cost ranges from nearly nothing — a can of sensor cleaner and a found hose — to a few hundred for a gasket or pump job. This is also the code where a scanner's live fuel-trim readings genuinely earn their keep. See P0171.

4 & 5. P0442 and P0455 — Small and large evap leaks

These two are siblings, so they share an entry — both mean the same thing in one sentence: the sealed system that stores gasoline vapors is leaking, slightly (P0442) or substantially (P0455). The most common real cause of the large-leak code is gloriously mundane: a gas cap left loose after a fill-up, or one with a worn seal. Tighten it until it clicks and the light often clears itself over a few drives. The small-leak code is the more annoying sibling — a pinhole in a hose, a weeping seal, or a sticky purge or vent valve — and finding it usually takes a shop's smoke machine. Cost: potentially zero, a replacement cap at worst pocket money, a valve or hose modest, and the main expense is often the diagnostic time to locate a small leak rather than the part itself. Neither code is urgent; you are leaking fumes, not safety.

6. P0128 — Engine running too cool

In one sentence: the engine is taking too long to warm up, or never reaches proper operating temperature. The most common real cause, by a wide margin, is a thermostat stuck open — it fails in the safe direction, letting coolant circulate constantly so the engine runs cold. Less often it is a faulty coolant-temperature sensor or a low coolant level. The symptoms are subtle: weak cabin heat in winter and fuel economy that quietly worsens, because a cold engine runs rich. Cost is on the gentle side — the thermostat itself is a cheap part — but labor varies enormously depending on where the engineers buried it, from twenty minutes on top of the engine to hours behind the accessory drive. See P0128.

7. P0301 — Misfire in cylinder 1

In one sentence: one specific cylinder — number one — is misfiring while the rest run fine. That specificity is a gift, because it narrows the suspects to what serves that cylinder alone: its spark plug, its coil, its injector, or its compression. The most common real cause is the plug or coil, and there is a classic diagnostic trick: swap the coil to a neighboring cylinder, clear the code, and see if the misfire follows the coil (the same logic works for plugs and, with more effort, injectors). Cost is cheap if it is a plug, moderate for a coil, more for an injector, and if compression testing reveals a mechanical problem — a burned valve, worn rings — you are into serious-repair territory. The same flashing-light warning from P0300 applies here — a hard single-cylinder misfire will flash the light and cook the converter just as effectively as a scattered one. Full write-up at P0301.

8. P0135 — Oxygen sensor heater circuit (bank 1, sensor 1)

In one sentence: the built-in heater that gets your upstream oxygen sensor working quickly after a cold start has an electrical fault. Unlike most sensor codes, this one usually does mean the sensor — the heater element inside burns out with age, and the fix is a new sensor. The exceptions worth checking first are a blown fuse or damaged wiring to the sensor, both cheaper. The car remains drivable, but it runs on a pre-programmed rich mixture until the sensor warms up on its own, costing fuel and working the converter harder every cold start. Cost is moderate: the sensor is a mid-priced part and usually accessible, so labor tends to be reasonable. See P0135.

9. P0446 — Evap vent control circuit

In one sentence: the valve that lets fresh air in and out of the fuel-vapor system is not responding the way the computer expects. The most common real cause is the vent valve itself sticking or failing, and on plenty of trucks and SUVs the true villain is simpler still — the vent line sits low and inhales dust and debris until it clogs. Wiring damage to the valve is the other regular suspect, especially where the harness runs near a wheel well. Like its evap siblings above, this is a no-urgency code: the part is typically inexpensive and often easy to reach near the charcoal canister, so cost usually stays in the low-to-moderate range unless diagnosis drags. Details at P0446.

10. P0011 — Camshaft timing over-advanced (bank 1)

In one sentence: the system that adjusts valve timing on the fly is holding the camshaft in the wrong position. Variable valve timing runs on pressurized engine oil, so the most common real cause is the least glamorous one: oil that is low, old, or the wrong viscosity, starving or gumming up the actuator. After that comes a sticking VVT solenoid with a clogged screen, and only then actual hardware — a worn phaser or a stretched timing chain. That ordering makes P0011 the rare code where the first step is an oil change, which is also why cost spans the widest range on this list: from the price of an oil service, to a solenoid at moderate cost, to open-the-front-of-the-engine timing work that rivals a converter bill. Start cheap, in order. See P0011.

Why the same code costs different amounts on different cars

You may have noticed every cost above came with a hedge. That is honesty, not evasion. A trouble code is standardized across every brand; the repair it leads to is anything but. Four things move the bill:

• Parts placement. The same thermostat is a twenty-minute job on one engine and a three-hour excavation on another. Labor hours, not the part, dominate many of these repairs.
• Parts pricing. A catalytic converter for a common sedan and one for a low-volume import can differ by a factor of several, before you even choose between factory and aftermarket.
• One code, many causes. P0420 might need a sensor or a converter; P0011 might need an oil change or a timing chain. Which suspect is guilty on your car decides the price more than the code does.
• Labor rates. The same job priced in a small town and a big city can differ by half again as much, purely on the shop's hourly rate.

This is exactly why generic code-cost tables disappoint: the useful answer is always specific to your car. When you have a code in hand, RedlineAi takes it — along with your exact make, model, mileage, and symptoms — and returns the likely causes ranked for your vehicle, how urgent it really is, and a fair cost range for your repair, not a national average. Read the code, check the cheap causes first, and price the fix for the car you actually drive.