Number Check-engine answers, priced honestly
The light is on. Before you pay anyone: decode the code free, see what the repair should cost in your state, and learn the traps that turn a $172 fix into a $1,511 one. Four tools, guides for the 150 most common codes, and a lookup that answers all 9,533.
OBD2 Code Lookup
All 9,533 generic fault codes — definition, ranked causes, severity, can-you-drive — answered instantly.
Repair Cost Estimator
Parts and labor ranges for 28 common repairs, adjusted for your state, vehicle class and shop type.
Maintenance Cost Planner
Your next two years of scheduled services, mapped from your odometer — with the cost of each.
Salvage Value Estimator
What a wrecked, flooded or totaled car is worth as salvage — plus the insurer total-loss math.
Car Total Cost of Ownership
Five-year ownership cost: payments, depreciation, fuel, insurance, upkeep — per year and per mile.
Repair vs Replace Calculator
Cost per added year for the repair vs the replacement, with the 50%-of-value rule as a guideline.
EV vs Gas Calculator
Purchase premium after incentives vs fuel and maintenance savings — the break-even year.
Buy vs Lease Car Calculator
True monthly cost of leasing vs buying, with the resale equity leasing never gives you.
What a check engine light really costs
The average check-engine repair in the US reached $554 in the CarMD Vehicle Health Index 2026 (39M+ vehicles, calendar-2025 data) — a record, driven by labor costs up 51% and parts up 23% over the index's history. But the average hides the spread: the ten most common repairs run from a $172 EVAP purge valve to a $1,511 catalytic converter, and the difference between them is usually diagnosis, not luck. A code names a system; the fix depends on which cause on the ranked list is yours.
| # | Most common check-engine repairs (CarMD Index 2026) | Typical codes | National average |
|---|---|---|---|
| 1 | Replace catalytic converter | P0420 / P0430 | $1,511 |
| 2 | Replace oxygen sensor(s) | P0135 – P0167 family | $287 |
| 3 | Replace ignition coil(s) and spark plug(s) | P0300 – P0308 | $480 |
| 4 | Replace mass air flow sensor | P0101 – P0103 | $346 |
| 5 | Replace ignition coil | P0350-family / misfire codes | $256 |
| 6 | Replace EVAP purge valve / solenoid | P0443 | $172 |
| 7 | Replace fuel injector(s) | P0200 – P0208 | $572 |
| 8 | Replace ABS wheel speed sensor | C0035-family | $314 |
| 9 | Replace thermostat | P0128 | $324 |
| 10 | Replace spark plug(s) | P0300 – P0308 | $299 |
Source: CarMD Vehicle Health Index 2026, published April 2026. Averages are parts + labor across all vehicles; your car and state will differ — that is what the estimator is for.
The 25 most common codes
| Rank | Code | What it is | Most likely fix |
|---|---|---|---|
| 1 | P0420 | Catalyst System Efficiency Below Threshold Bank 1 | Oxygen sensor — downstream (post-cat) |
| 2 | P0430 | Catalyst System Efficiency Below Threshold Bank 2 | Oxygen sensor — downstream (post-cat) |
| 3 | P0136 | O2 Sensor Circuit Bank 1 Sensor 2 | Oxygen sensor — downstream (post-cat) |
| 4 | P0141 | O2 Sensor Heater Circuit Bank 1 Sensor 2 | Diagnosis first |
| 5 | P0135 | O2 Sensor Heater Circuit Bank 1 Sensor 1 | Oxygen sensor — upstream (pre-cat) |
| 6 | P0155 | O2 Sensor Heater Circuit Bank 2 Sensor 1 | Diagnosis first |
| 7 | P0300 | Random/Multiple Cylinder Misfire Detected | Ignition coils + spark plugs (full set) |
| 8 | P0301 | Cylinder 1 Misfire Detected | Ignition coil — single cylinder |
| 9 | P0302 | Cylinder 2 Misfire Detected | Ignition coils + spark plugs (full set) |
| 10 | P0303 | Cylinder 3 Misfire Detected | Ignition coils + spark plugs (full set) |
| 11 | P0304 | Cylinder 4 Misfire Detected | Ignition coils + spark plugs (full set) |
| 12 | P0101 | Mass or Volume Air Flow Sensor A Circuit Range/Performance | Mass air flow (MAF) sensor |
| 13 | P0102 | Mass or Volume Air Flow Sensor A Circuit Low | Mass air flow (MAF) sensor |
| 14 | P0443 | EVAP System Purge Control Valve A Circuit | EVAP purge valve / solenoid |
| 15 | P0201 | Cylinder 1 Injector A Circuit/Open | Fuel injector service / replacement set |
| 16 | P0305 | Cylinder 5 Misfire Detected | Ignition coils + spark plugs (full set) |
| 17 | P0306 | Cylinder 6 Misfire Detected | Ignition coils + spark plugs (full set) |
| 18 | C0035 | Right Front Wheel Speed Sensor Supply | ABS wheel speed sensor |
| 19 | C0040 | Brake Pedal Switch A | ABS wheel speed sensor |
| 20 | P0128 | Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) | Thermostat + housing + coolant |
| 21 | P0125 | Insufficient Coolant Temperature for Closed Loop Fuel Control | Thermostat + housing + coolant |
| 22 | P0442 | EVAP System Leak Detected (small leak) | EVAP leak diagnosis (smoke test) + gas cap |
| 23 | P0455 | EVAP System Leak Detected - Large Leak | EVAP leak diagnosis (smoke test) + gas cap |
| 24 | P0171 | System Too Lean Bank 1 | Intake / vacuum leak repair (gaskets, hoses) |
| 25 | P0174 | System Too Lean Bank 2 | Intake / vacuum leak repair (gaskets, hoses) |
Browse the code library by system
Every guide follows the same standard: ranked causes, can-you-drive verdict, misdiagnosis traps, DIY difficulty, shop-vs-DIY costs, and the warranty check where one exists.
Engine misfire (7 of the top 150 · P0300–P0316)
One or more cylinders failing to fire — the only family that can flash the light and destroy a catalyst in minutes.
Catalyst & exhaust efficiency (2 of the top 150 · P0420–P0433)
The converter is no longer cleaning the exhaust to standard — the most common check-engine repair in America.
Oxygen & exhaust sensors (18 of the top 150 · P0130–P0167 · P2195–P2273)
The sensors that measure the exhaust — heater circuits, slow response, stuck signals.
Fuel trim & air metering (17 of the top 150 · P0100–P0114 · P0170–P0175 · P2187–P2188)
Too much air or too little fuel: lean and rich codes, MAF and intake faults.
EVAP — fuel vapor system (23 of the top 150 · P0440–P0459 · P0496–P0499 · P2400s)
Vapor leaks from pinhole to missing gas cap. Harmless to drive on, fatal to emissions tests.
EGR system (7 of the top 150 · P0400–P0409)
Exhaust gas recirculation flow faults — usually carbon, occasionally the valve.
Cooling system (9 of the top 150 · P0115–P0128 · P2181)
Thermostat and coolant-temperature faults that quietly eat fuel economy and emissions.
Variable valve timing (15 of the top 150 · P0010–P0029 · P2088–P2095)
Cam timing and oil-control faults — very often an oil problem before it is a parts problem.
Engine position & speed sensors (4 of the top 150 · P0335–P0349 · P0500–P0503)
Crank, cam and speed sensors — the engine's sense of where it is.
Idle & throttle control (15 of the top 150 · P0505–P0513 · P0600s · P2100s)
Idle speed, electronic throttle and control-module codes.
Transmission (20 of the top 150 · P0700–P0799 · P2700s)
Shift, pressure, solenoid and ratio faults — from fluid level to a failing gearbox.
ABS & brakes (C-codes) (2 of the top 150 · C0035–C0051 +)
Wheel-speed and brake-system faults. The brakes still stop the car; the anti-lock safety net is off.
How the top 150 were chosen — and where the data comes from
Ranks 1–23 mirror the repair families in CarMD's published top-10 most common check-engine repairs (the converter, oxygen-sensor, ignition, MAF, EVAP, injector, ABS-sensor and thermostat families, with the specific codes those repairs answer to). Ranks 24–80 are the long-standing most-reported generic codes across CarMD Index editions and standard trade references; ranks 81–150 complete system coverage (EVAP, timing, transmission, idle and network faults a driver plausibly meets). Per-code percentages are claimed only where a published source states them. Code facts, causes and symptoms come from OBDex (CC0 1.0); costs are our model with CarMD averages as the cross-check; state labor uses the BLS OEWS May 2025 wage index. The next 350 guides ship only if this set earns it — at least 70% indexed with real impressions inside 30 days, judged by the network's daily SEO loop.
Auto repair questions
Where do the repair prices come from?
Two published sources, both cited wherever they are used: our cost model is built from national parts/labor ranges and adjusted by the BLS OEWS state wage index for technicians; the cross-check averages come from the CarMD Vehicle Health Index 2026, computed from 39M+ vehicles. Nothing here is scraped from another estimator, and no shop pays to appear.
Why only 150 code guides when there are 9,533 codes?
Because almost all real-world check-engine lights come from a few hundred codes. The top 150 get full guides; the lookup tool answers every other code from the same dataset, honestly labeled. More guides are added when the lookup's own search data shows demand — pages earn their existence.
Can I really drive with the light on?
It depends on the code — which is the whole point of the severity line on every guide. Emissions faults usually drive fine for weeks; a flashing light or a misfire code means stop soon, because the catalyst is being damaged in real time. When in doubt, the guide for your exact code says which case you are in.
Is the federal emissions warranty real?
Yes — federal law requires manufacturers to cover the catalytic converter and engine computer for 8 years/80,000 miles on 1995-and-newer cars, with longer coverage for many parts on PZEV vehicles in CARB states. A surprising share of the most expensive check-engine repairs are free at a dealership inside that window; every emissions guide here says so.
Do you list or recommend repair shops?
No. We publish data and tools, not shop referrals — no listings, no sponsored placements, and no lead selling. Take the ranges here to any shop and compare their itemized quote against them.