Your check engine light came on and a scanner gave you a code like P0420. Here’s what the most common OBD-II codes mean, what usually causes them, how urgent they are and what the fix typically costs, so you can talk to a shop with confidence.
New to this? Start with our check engine light guide, then come back here to look up your code.
Common OBD-II codes
| Code | What it means | Common causes | Urgency | Typical fix cost |
|---|---|---|---|---|
| P0011 | Camshaft position timing over-advanced (Bank 1) | Low or dirty oil, faulty oil control (VVT) valve, stretched timing chain | Medium | Diagnosis first ($100–$200) |
| P0016 | Crankshaft/camshaft position correlation (Bank 1) | Stretched timing chain, jumped timing belt, faulty cam or crank sensor | High | Diagnosis first ($100–$200); see timing belt costs |
| P0101 | Mass airflow (MAF) sensor range/performance | Dirty MAF sensor, air leak after the sensor, clogged air filter | Low–Medium | Cleaning is cheap; replacement varies |
| P0128 | Coolant temperature below thermostat regulating temperature | Thermostat stuck open, faulty coolant temperature sensor | Low | Diagnosis first ($100–$200) |
| P0133 | O2 sensor slow response (Bank 1, Sensor 1) | Aging oxygen sensor, exhaust leak | Low–Medium | O2 sensor about $150–$270 |
| P0135 | O2 sensor heater circuit (Bank 1, Sensor 1) | Failed sensor heater, damaged wiring, blown fuse | Low | O2 sensor about $150–$270 |
| P0141 | O2 sensor heater circuit (Bank 1, Sensor 2) | Failed downstream sensor heater, wiring | Low | O2 sensor about $150–$270 |
| P0171 | System too lean (Bank 1) | Vacuum leak, dirty MAF sensor, weak fuel pump or clogged filter, injectors | Medium | Diagnosis first ($100–$200) |
| P0172 | System too rich (Bank 1) | Leaking injector, faulty MAF or O2 sensor, high fuel pressure | Medium | Diagnosis first ($100–$200) |
| P0174 | System too lean (Bank 2) | Same causes as P0171 on the other cylinder bank | Medium | Diagnosis first ($100–$200) |
| P0300 | Random or multiple cylinder misfire | Worn spark plugs, ignition coils, vacuum leak, fuel delivery problem | High if the light flashes | Spark plugs about $150–$400 |
| P0301–P0308 | Misfire in cylinder 1–8 | Spark plug, coil or injector on that cylinder; low compression | High if the light flashes | Spark plugs about $150–$400; coils extra |
| P0325 | Knock sensor 1 circuit (Bank 1) | Faulty knock sensor, damaged wiring or connector | Medium | Diagnosis first ($100–$200) |
| P0335 | Crankshaft position sensor A circuit | Failed crank sensor, wiring; can cause stalling or no-start | High | Diagnosis first ($100–$200) |
| P0340 | Camshaft position sensor A circuit (Bank 1) | Failed cam sensor, wiring; may cause hard starting | Medium–High | Diagnosis first ($100–$200) |
| P0401 | Exhaust gas recirculation (EGR) flow insufficient | Clogged EGR valve or passages, faulty EGR valve | Low–Medium | Diagnosis first ($100–$200) |
| P0420 | Catalyst system efficiency below threshold (Bank 1) | Worn catalytic converter, faulty O2 sensor, exhaust leak, past misfires | Medium | Converter about $1,500–$2,500; check sensors first |
| P0430 | Catalyst system efficiency below threshold (Bank 2) | Same causes as P0420 on the other bank | Medium | Converter about $1,500–$2,500; check sensors first |
| P0440 | Evaporative emission (EVAP) system malfunction | Purge or vent valve, cracked hoses, gas cap | Low | Diagnosis first ($100–$200) |
| P0442 | EVAP system small leak detected | Worn gas cap seal, cracked hose, leaking purge valve | Low | Try a new gas cap ($15–$40) first |
| P0446 | EVAP vent control circuit | Faulty vent valve or solenoid, blocked vent | Low | Diagnosis first ($100–$200) |
| P0455 | EVAP system large leak detected | Loose or missing gas cap, disconnected hose | Low | Often free: tighten or replace the cap |
| P0456 | EVAP system very small leak detected | Gas cap seal, tiny hose crack | Low | Try a new gas cap ($15–$40) first |
| P0457 | EVAP leak detected (fuel cap loose or off) | Gas cap not tightened or missing | Low | Often free: tighten or replace the cap |
| P0505 | Idle air control system malfunction | Dirty throttle body, faulty idle control valve, vacuum leak | Low–Medium | Diagnosis first ($100–$200) |
| P0700 | Transmission control system malfunction | The transmission computer has stored its own code; scan it next | Medium–High | See transmission costs |
| P0715 | Input/turbine speed sensor circuit | Faulty transmission speed sensor, wiring | Medium | See transmission costs |
Costs are typical US ranges for parts and labor and are estimates only. A code tells you which system has a problem, not exactly which part failed. A proper diagnosis ($100–$200 at most shops) is usually cheaper than guessing.
How to read an OBD-II code
Every code has five characters. Each one tells you something:
| Position | Example (P0420) | What it tells you |
|---|---|---|
| 1st: letter | P | System: P = powertrain (engine and transmission), B = body, C = chassis, U = network/communication |
| 2nd: digit | 0 | 0 = generic code (same meaning on all cars); 1 = manufacturer-specific code |
| 3rd: digit | 4 | Subsystem: 1–2 fuel and air, 3 ignition/misfire, 4 emissions controls, 5 speed and idle control, 6 computer and outputs, 7–9 transmission |
| 4th–5th: digits | 20 | The specific fault within that subsystem |
What “Bank 1” and “Sensor 1” mean
- Bank 1 is the side of the engine with cylinder number 1. Bank 2 is the other side, and only exists on V6, V8 and other V-shaped engines.
- Sensor 1 is the oxygen sensor before the catalytic converter (upstream). Sensor 2 is after the converter (downstream).
What the urgency ratings mean
| Rating | What to do |
|---|---|
| Low | Safe to drive normally. Get it checked when convenient, and before an emissions test. |
| Medium | Get it diagnosed within a week or two. Ignoring it can lead to bigger repairs or worse fuel economy. |
| High | Get it checked soon. If the check engine light is flashing, or the car runs badly or stalls, limit driving to avoid damage. |
How to get your code read
- Free: most auto parts stores will read your code at no charge.
- About $20–$40: buy a basic OBD-II scanner. It plugs into the port under the dashboard on any car sold in the US since 1996.
- About $100–$200: a professional diagnosis, which tests the system to find the actual failed part.
Related guides
- Check engine light: what it means and what it costs
- Repair cost estimator
- Transmission repair and replacement cost
Code definitions follow the standard SAE J2012 generic OBD-II codes. Manufacturer-specific codes (with a “1” as the second character) can mean different things on different brands. Costs are estimates for general guidance only; see our disclaimer.