OBD2 Trouble Code Lookup

The 15 most common OBD2 codes and what to do

A trouble code (DTC) is always a letter plus four digits: P for powertrain, B body, C chassis, U network. These powertrain codes are among the ones drivers and mechanics see most often.

• P0300 — Random/multiple cylinder misfire. Severity: high. Causes: worn spark plugs, faulty ignition coil, bad fuel injector. Fix plugs and coils first.
• P0301–P0308 — Cylinder N misfire. Same as P0300 but identifies the specific cylinder. Easier to diagnose: swap the coil on that cylinder first.
• P0420 — Catalyst system efficiency below threshold (Bank 1). Severity: medium. Causes: aging catalytic converter, O2 sensor fault, exhaust leak. Often lasts months before causing drivability issues. Full P0420 guide →
• P0430 — Catalyst system efficiency below threshold (Bank 2). Same as P0420 but for the second bank on V6/V8 engines.
• P0171 — System too lean (Bank 1). Severity: medium. Causes: vacuum leak, dirty MAF sensor, weak fuel pump. Fix vacuum leaks first — they’re free.
• P0174 — System too lean (Bank 2). As above on Bank 2. Often occurs alongside P0171 on V-engines.
• P0101 — MAF sensor range/performance. Severity: medium. Causes: dirty or failed mass airflow sensor. Clean the sensor with MAF cleaner spray before replacing.
• P0128 — Coolant temperature below thermostat regulating temperature. Severity: low/medium. Cause: failed thermostat stuck open. Engine runs rich until it warms up; fuel economy drops.
• P0401 — EGR flow insufficient. Severity: low-medium. Cause: clogged EGR valve or passages. Often cleanable without replacement.
• P0455 — EVAP system large leak detected. Severity: low. Very common cause: loose or cracked fuel cap. Tighten or replace the fuel cap first.
• P0442 — EVAP system small leak detected. As above. Check fuel cap, then EVAP hoses and purge valve.
• P0507 — Idle control system RPM high. Severity: low. Causes: vacuum leak, dirty throttle body. Clean the throttle body with aerosol cleaner.
• P0340 — Camshaft position sensor circuit malfunction. Severity: high. Can cause no-start. Replace the sensor; check the reluctor ring.
• P0011 — Camshaft position ‘A’ timing — over-advanced (Bank 1). Common on VVT engines with low or dirty oil. Change oil first.
• P0562 — System voltage low. Severity: medium. Causes: weak battery, bad alternator, loose connections. Test the battery and charging system.

Other code families

• B (Body) — airbag, climate, power windows. Usually shop-only.
• C (Chassis) — ABS, traction control, suspension.
• U (Network) — communication faults between modules. Often secondary codes; fix the primary first.

Reading codes with an OBD2 head-up display

An OBD2 scanner reads and clears stored codes. Some OBD2 head-up displays can also show fault alerts and live data such as coolant temperature and voltage while you drive. The wiiyii P6’s listing, for example, includes reading data streams and clearing fault codes, on 2007+ gas cars on its fitment list. A HUD is not a replacement for a proper diagnosis when the check-engine light is on.

As an Amazon Associate, HUDScreen earns from qualifying purchases. Disclosure.

Frequently asked questions

What is an OBD2 trouble code?

It is a standardized code (like P0420) that your car stores when it detects a fault. A scanner or some OBD2 head-up displays can read it.

What is the most common OBD2 code?

Misfire codes (P0300–P0308), catalyst efficiency codes (P0420/P0430), lean codes (P0171/P0174) and EVAP leak codes (P0455/P0442) are among the most frequent. For EVAP codes, check that the fuel cap is tight first.

Can I drive with an OBD2 trouble code?

It depends on the code. A loose fuel cap (P0455) is not urgent. A misfire (P0300) or a camshaft sensor fault (P0340) should be checked promptly because it can damage the engine or catalytic converter. A flashing check-engine light means stop driving hard and get it checked.

How do I clear OBD2 codes without a scanner?

Disconnecting the battery for 15–30 minutes clears most codes, but it also resets the emissions readiness monitors, which can cause a failed smog check until they complete again. A scanner clears codes more cleanly; fix the cause first or the code will return.

Can a head-up display read these codes?

Some OBD2 head-up displays show fault alerts and can clear codes, but they show far less detail than a scanner. Use a scanner or this lookup for the full meaning of a code.