OBD-II Trouble Codes List: P0300, P0420 and Common Check Engine Codes
Quick Answer: P0300 means random misfire, P0420 means catalyst efficiency below threshold, P0171/P0174 mean lean conditions bank 1/2, and P0455 means a large EVAP leak (usually the gas cap). Codes starting P0 are universal across all makes; P1 codes are manufacturer-specific.
This lookup table covers the codes that account for most check-engine-light visits, what each means, how urgent it is, and the estimated fix cost – with a link to the full diagnosis guide where we have one.
Common OBD-II Trouble Code Lookup Table
| Code | Meaning | Urgency | Typical Fix Cost |
|---|---|---|---|
| P0300 | Random / multiple cylinder misfire | High – stop driving if flashing | $100-$700 (plugs, coils, or fuel) |
| P0301-P0308 | Misfire in specific cylinder | High | $50-$400 (coil/plug/injector) |
| P0420 | Catalyst efficiency below threshold | Medium | $100 (O2 sensor) – $1,500 (cat) |
| P0171 / P0174 | System too lean (bank 1 / bank 2) | Medium | $20 (vacuum hose) – $400 (MAF, pump) |
| P0172 / P0175 | System too rich (bank 1 / bank 2) | Medium | $100-$600 (injector, regulator, sensor) |
| P0455 | EVAP large leak detected | Low | $10 (gas cap) – $150 (purge valve) |
| P0442 | EVAP small leak | Low | $10-$150 |
| P0128 | Coolant thermostat below regulating temp | Low-Medium | $150-$350 (thermostat) |
| P0300-family + P0440 | See misfire + EVAP rows | – | – |
| P0101 | MAF sensor circuit range/performance | Medium | $15 (clean) – $250 (replace) |
| P0401 | EGR insufficient flow | Medium | $150-$600 (clean or replace EGR) |
| P0402 | EGR excessive flow | Medium | $150-$600 |
| P0011 / P0014 | Camshaft position timing over-advanced | Medium-High | $120 (oil change + VCT solenoid) – $1,000+ |
| P0446 | EVAP vent control malfunction | Low | $50-$200 (vent valve) |
| P0505 | Idle control system malfunction | Medium | $100-$400 (IAC/throttle body) |
| P0700 | Transmission control system fault | High – see trans shop | varies widely |
| P2181 | Cooling system performance | Medium | $150-$500 (thermostat/pump/fan) |
| P2096 / P2098 | Post-catalyst fuel trim lean | Medium | $100-$600 |
How to Read the Code Format
The first letter identifies the system: P = powertrain, B = body, C = chassis, U = network. The second digit tells you if the code is generic (0) or manufacturer-specific (1). The remaining digits point to the subsystem – 03xx is misfires, 04xx is emissions and EVAP, 01xx is fuel and air metering.
Freeze Frame Data Is Your Friend
Any cheap OBD-II scanner shows the conditions when the code set: RPM, load, coolant temp, speed. P0171 that appears only at cold start points somewhere different than one that appears at highway cruise. Read the freeze frame before buying parts.
Full Diagnosis Guides
We publish deep-dive diagnosis for the most common codes and symptoms:
Frequently Asked Questions
Can I drive with a check engine light on?
A steady light with normal driving behavior: yes, short-term. A flashing light means active misfire that is destroying the catalytic converter – stop driving and diagnose now.
Will the code clear itself?
Most codes clear after 3 consecutive good drive cycles, but the underlying problem remains if it is not fixed. Clearing with a scanner without fixing the cause just resets the countdown.
What scanner should I buy?
A $25 Bluetooth OBD-II dongle with a phone app reads and clears codes and shows freeze frame data. Professional bi-directional tools are only needed for actuator tests and module work.
Does a code tell me exactly which part failed?
No. A code names the circuit or system that reported a problem, not the failed part. An O2 sensor code can be caused by a vacuum leak. Diagnose before replacing – that is the entire point of this site.
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