OBD-II glossary: acronyms and terms

Plain definitions of the acronyms and terms you meet when reading OBD-II codes: MIL, DTC, PCM, MAF, EVAP, EGR, freeze frame, readiness, pending codes and more.

OBD-II uses a lot of abbreviations. SAE J1930 standardizes the terms and acronyms that OBD-II information must use, so the same name means the same part on every make. This glossary explains the ones you meet most when reading codes and live data, grouped by topic.

Systems, rules and standards

Term	Meaning
OBD	On-board diagnostics. The vehicle's own system for detecting faults in emission-related parts. Today "OBD" and "OBD-II" are used to mean the same thing.
OBD I	The first generation, required by California from the 1988 model year. Limited monitoring, and connectors and codes differed by maker.
OBD-II	The second generation, on all US cars and light trucks from the 1996 model year (diesels from 1997). See what is OBD-II.
EOBD	European on-board diagnostics, required by Directive 98/69/EC on petrol cars up to 2,500 kg from 2001 and diesel cars up to 2,500 kg from 2004; heavier cars later. Uses the same connector and code format.
CARB	California Air Resources Board. Wrote the OBD I and OBD II rules. The current OBD II rule is title 13, California Code of Regulations, section 1968.2.
EPA	US Environmental Protection Agency. Sets the federal OBD rules (40 CFR 86.1806) and the rules for OBD checks in inspection programs (40 CFR 85.2222).
SAE	SAE International, the engineering body that publishes the J-standards used by OBD-II.
ISO	International Organization for Standardization. Publishes the ISO 15031 series, equivalent to the SAE OBD standards, and the communication protocols.
I/M	Inspection and maintenance: a state or local emissions testing program.
J1962	The SAE standard for the 16-pin diagnostic connector (ISO 15031-3). See the pinout guide.
J1979	The SAE standard for diagnostic test modes and data (ISO 15031-5).
J2012	The SAE standard for trouble code definitions (ISO 15031-6).
J1930	The SAE standard for diagnostic terms, definitions, abbreviations and acronyms (ISO/TR 15031-2).
J1978	The SAE standard for a generic OBD-II scan tool (ISO 15031-4).
DLC	Data link connector: the OBD-II socket.
ALDL	Assembly Line Diagnostic Link. General Motors' pre-OBD-II diagnostic protocol, originally on a 12-pin connector. Some 1994–1995 GM vehicles used a 16-pin socket that looks like the OBD-II one but still spoke ALDL, which OBD-II tools cannot read.
GVWR	Gross vehicle weight rating. Used to decide which OBD rules apply to larger trucks.
FTP	Federal Test Procedure: the US laboratory emissions test. OBD fault thresholds are set as multiples of its emission standards.

Communication

Term	Meaning
CAN	Controller Area Network (ISO 15765-4). Required for OBD-II on US cars and light and medium-duty trucks from the 2008 model year (heavy-duty trucks use SAE J1939). Uses pins 6 and 14.
KWP2000	Keyword Protocol 2000 (ISO 14230-4). Uses the K-line on pin 7.
ISO 9141-2	An older protocol that also uses the K-line on pin 7.
J1850 PWM / VPW	Two older SAE protocols. VPW uses pin 2; PWM uses pins 2 and 10.
K-line, L-line	The single-wire data lines used by ISO 9141-2 and ISO 14230-4 (pins 7 and 15).
Mode / service	A type of request a scan tool sends under J1979. US emissions tests use Mode $01 PID $01 for monitor status and Mode $03 for stored codes.
PID	Parameter ID: the number that identifies one item of data within a mode, such as engine speed.
Generic scan tool	A tool built to J1978 that reads the standard OBD-II data from any compliant vehicle.
Enhanced scan tool	Any scan tool that goes beyond a generic J1978 tool, usually to read a maker's own codes and data.

Codes, the light and readiness

Term	Meaning
DTC	Diagnostic trouble code, such as P0420. See how to read an OBD-II code.
MIL	Malfunction indicator lamp: the check engine light, often labelled "Check Engine" or "Service Engine Soon". Blinks for a misfire that can damage the catalytic converter.
Pending code	A code stored when a fault is first detected, usually on one driving cycle, before the MIL comes on.
Confirmed code	A code stored when the fault is confirmed, usually on the second driving cycle in which it is detected. Turns the MIL on.
Permanent code	A confirmed code that is currently commanding the MIL on, kept in memory that a scan tool or battery disconnect cannot erase. Phased in from 2010.
Freeze frame	A snapshot of engine data saved when a code is stored.
Driving cycle	An engine start and the following engine shut-off, including the engine-off time until the next start.
Warm-up cycle	Enough driving for coolant temperature to rise by at least 40°F (22°C) from start-up and reach at least 160°F (71°C); 140°F (60°C) for diesels. A confirmed code erases itself after 40 warm-up cycles without the fault.
Monitor	A self-test the computer runs on one system, such as the catalyst or the evaporative system.
Continuous monitor	A monitor that runs all the time (misfire, fuel system, comprehensive components). Always shows complete.
Readiness status	Whether each monitor is "complete" or "not complete" since codes were last cleared. Checked by emissions tests. See drive cycles and readiness.
Drive cycle	A driving pattern that lets the monitors run. Each maker publishes its own.
Limp-home mode	A default operating mode the computer switches to after some faults. Under OBD-II, entering it turns on the MIL within 10 seconds if it can affect emissions.
Rationality check	A test of whether a sensor's reading, while within its normal range, makes sense compared with the other information the computer has.
Circuit continuity	A test for open circuits, shorts and out-of-range signals on a sensor or output circuit.

Control modules

Term	Meaning
ECU	Electronic control unit: any control module.
ECM	Engine control module.
PCM	Powertrain control module: a module that controls both engine and transmission.
TCM	Transmission control module.
NVRAM	Non-volatile memory: memory that keeps its contents with the battery disconnected. Permanent codes are stored here.

Sensors and data

Term	Meaning
MAF	Mass air flow sensor: measures the air entering the engine. Code example: P0101.
MAP	Manifold absolute pressure sensor: measures pressure in the intake manifold. Example: P0106.
IAT	Intake air temperature sensor. Example: P0113.
ECT	Engine coolant temperature sensor. Example: P0118.
TP / TPS	Throttle position sensor. Example: P0122.
CKP	Crankshaft position sensor. Example: P0335.
CMP	Camshaft position sensor. Example: P0340.
VSS	Vehicle speed sensor. Example: P0500.
KS	Knock sensor. Example: P0325.
O2S / HO2S	Oxygen sensor / heated oxygen sensor. See the oxygen sensor guide.
A/F sensor	Air-fuel ratio sensor: a wideband oxygen sensor that measures the actual mixture.
Bank 1 / Bank 2	The side of the engine with cylinder 1 is bank 1; the other side is bank 2.
Calculated load	Engine load as a percentage, worked out by the computer. One of the values every OBD-II vehicle must report.
Fuel trim	The correction the computer applies to fuel delivery to hold the right mixture. Short-term trim reacts quickly; long-term trim is the learned correction.
Open loop / closed loop	Fuel control status. In closed loop the computer uses the oxygen sensor to correct the mixture; in open loop it does not, for example on a cold engine.
Lambda (λ)	The ratio of the actual air-fuel mixture to the chemically correct one. λ = 1 is correct, below 1 is rich, above 1 is lean.
Stoichiometric	The chemically correct air-fuel ratio, λ = 1: about 14.7 kg of air per kg of gasoline.

Emission control systems

Term	Meaning
CAT / TWC	Catalytic converter / three-way catalyst: turns hydrocarbons, carbon monoxide and nitrogen oxides into carbon dioxide, water and nitrogen.
EGR	Exhaust gas recirculation: feeds some exhaust gas back into the intake. Example: P0401.
EVAP	Evaporative emission system: holds fuel vapor from the tank and later purges it into the engine to be burned. Example: P0442.
PCV	Positive crankcase ventilation: routes crankcase gases back into the intake.
AIR	Secondary air injection: pumps outside air into the exhaust.
VVT	Variable valve timing.
DPF / PM filter	Diesel particulate filter: traps particulate matter (soot) from diesel exhaust.
NMHC	Non-methane hydrocarbons. A diesel NMHC catalyst is monitored by OBD-II.
NOx	Nitrogen oxides.
HC, CO	Hydrocarbons, carbon monoxide.

For the full list of trouble codes, see the code list.

OBD DashX reads and clears OBD-II trouble codes on Android with a Bluetooth ELM327 adapter.

Sources: SAE J1930 (as used in 13 CCR 1968.2 and 40 CFR 86.1806); 13 CCR 1968.2(c); 40 CFR 85.2222; EPA420-F-03-042; CARB OBD II fact sheet; Bosch