Car & OBD2 Glossary

On-Board Diagnostics, second generation. A standard diagnostic interface mandated on all US vehicles since 1996 and EU vehicles since 2001. Exposes a 16-pin port (usually under the driver-side dash) for reading trouble codes and live sensor data. See how to read OBD2 codes.

A 5-character code (like P0420) emitted by a vehicle's ECU when a sensor reports an out-of-range value. DTCs are read via an OBD2 scanner. First character is the system (P = powertrain, B = body, C = chassis, U = network); next four identify the specific fault.

The vehicle's central computer. Manages fuel injection, ignition timing, and emissions; stores DTCs when sensors fail. Also called PCM (Powertrain Control Module) on some vehicles.

The dashboard warning light (often engine-shaped) that comes on when the ECU stores a confirmed DTC. A solid CEL is non-emergency. A flashing CEL means a misfire severe enough to damage the catalytic converter; pull over and stop driving.

A sensor in the intake tract that measures the mass of air entering the engine. Dirty or failing MAFs often cause P0171 (lean) codes. Cleaning with MAF-specific cleaner is a $10 DIY; replacement is typically $50–$200.

A sensor that reads pressure inside the intake manifold. Used by the ECU alongside or instead of a MAF sensor to calculate engine load and fuel delivery.

A sensor in the exhaust that measures oxygen content, feeding the ECU closed-loop air/fuel data. Upstream O2 sensors (before the cat) control fuel trim. Downstream O2 sensors (after the cat) monitor catalytic converter health and are the usual culprits behind P0420.

The de-facto chipset standard used in consumer OBD2 scanners. Most $15–$40 Bluetooth OBD2 adapters are ELM327-based. Trackara works with any ELM327-compatible adapter.

A 17-character identifier unique to each vehicle. Encodes manufacturer, year, plant, and serial number. Use it to look up recalls, specs, and service history.

The paired US and international standards defining the OBD2 diagnostic communication protocol. Every OBD2 scanner speaks these protocols, which is why a cheap ELM327 adapter works on a Honda Civic and an F-150 alike.

The SAE standard defining the 5-character DTC format (P, B, C, U prefixes) and standardized powertrain DTCs. Every P0XXX code (generic powertrain) follows J2012; P1XXX codes are manufacturer-specific.

California's emissions regulatory body. CARB rules often set the US de-facto standard for OBD2 and emissions inspection readiness monitors.

Internal ECU self-tests (catalyst, O2 heater, EVAP, EGR, etc.) that must complete before a vehicle can pass emissions inspection. Clearing DTCs resets these, which is why clearing codes the day before a smog test is a bad idea.

A specific sequence of driving conditions (cold start, idle, cruise, deceleration) that the ECU uses to complete readiness monitors after codes are cleared. Varies by manufacturer; typically 30–60 minutes of mixed driving.

A snapshot of ECU sensor values (RPM, coolant temp, fuel trim, throttle position) captured at the exact moment a DTC was stored. Critical for diagnosing intermittent faults.

The ECU's running adjustment to fuel injection, expressed as a percentage. Short-term trim (STFT) reacts second-by-second; long-term trim (LTFT) reflects persistent adjustments. LTFT above +10% often triggers P0171 lean codes.

An engine design where piston and valve travel overlap. If the timing belt or chain breaks, pistons collide with open valves and usually destroy the engine. Most modern engines are interference designs, which is why timing belt replacement is non-negotiable.

A toothed rubber belt synchronizing crankshaft and camshaft rotation. Typical replacement interval: 60,000–100,000 miles depending on manufacturer. Non-negotiable on interference engines.

A metal chain performing the same role as a timing belt. Generally lasts the life of the engine with proper oil changes, though chain tensioner and guide failures do occur on some models.

An exhaust component that converts harmful emissions (HC, CO, NOx) into less harmful gases. A failing cat is the most common cause of P0420 codes. Replacement runs $400 to $2,500 depending on the vehicle.

The system that captures and burns fuel vapors from the gas tank. A loose gas cap is the #1 cause of P0440 / P0442 / P0455 EVAP codes. Always check the gas cap first.

Any engine that burns fuel to create power. The traditional gasoline or diesel engine. Contrast with EV (electric vehicle).

A vehicle powered by an electric motor and battery rather than an internal combustion engine. EVs don't produce traditional OBD2 DTCs but most still expose OBD2 data for state-of-charge, motor temperature, and drivetrain health.

A vehicle with both an ICE and an electric motor. Covered by OBD2 like any ICE vehicle, with additional hybrid-specific DTCs (P0A00-series).

Parts made by the same manufacturer as the vehicle, or by its contracted supplier. OEM parts cost more than aftermarket but often fit better and last longer. Contrast with aftermarket.

A manufacturer-issued document describing a known issue and recommended fix. Not a recall (not legally binding) but often very useful for diagnosing model-specific problems. NHTSA publishes TSBs for free.

Performing auto repair or maintenance yourself rather than paying a shop. Browse DIY guides.

Fuel economy measured as miles traveled per gallon of fuel consumed.

The US federal agency that manages vehicle safety recalls. NHTSA's free recall database is the authoritative source for all US recalls and is searchable by VIN.