OBD-II glossary
The terms that appear in diagnostic procedures, defined plainly. If a code page uses a word you don't recognise, it's almost certainly here.
- OBD-II
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The standardized on-board diagnostic system used on US-market light-duty gasoline vehicles from the 1996 model year.
OBD-II provides a diagnostic connector, communication methods and standardized emissions information. Coverage differs for other vehicle classes, fuels and markets. Generic codes coexist with manufacturer-specific information; the code label does not make the diagnostic procedure identical across vehicles.
- Diagnostic Trouble Code (DTC)
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A stored identifier for a detected fault or failed monitor.
A code identifies the system, circuit or condition being reported. It does not prove which component caused it. For example, P0420 reports a catalyst-efficiency result; diagnosis must establish why that result occurred.
- Freeze frame
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Operating data saved around a detected fault.
Available values may include engine speed, load, temperature and fuel trim. The trigger, available parameters and which record is retained vary by vehicle. Save this evidence before clearing codes so the original conditions can guide diagnosis.
- Fuel trim
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A correction the engine computer applies to its fuel calculation.
Short-term trim adjusts fueling during feedback control; long-term trim stores learned corrections. Positive trim generally adds fuel and negative trim removes it. Interpret values with operating mode, both trims, bank and manufacturer limits; one percentage is not a universal failure threshold.
- Bank 1 / Bank 2
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Bank 1 is the cylinder group containing cylinder number one.
Use the manufacturer cylinder numbering and bank layout. Many inline engines have one bank and many V or flat engines have two, but do not choose a component by assuming a particular left or right side.
- Upstream / downstream sensor
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Upstream is before the catalyst; downstream is after it in the specified exhaust path.
Upstream oxygen or air-fuel sensors commonly support mixture control, while downstream sensors commonly support catalyst monitoring. Some systems also use downstream feedback for fuel control. Sensor numbers, technology, part numbers and diagnostic interpretation depend on the vehicle layout.
- Drive cycle
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Operating conditions that allow a particular monitor to run.
A monitor may require specified temperatures, fuel level, speeds, loads or an engine-off period. Use the vehicle procedure and confirm the relevant monitor result. An incomplete monitor is not proof that a repair failed or succeeded.
- Readiness monitor
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A status indicating whether a supported emissions monitor has completed its check.
A scan tool can show supported monitors as complete or incomplete. Clearing diagnostic information commonly resets readiness. Inspection rules differ by jurisdiction and vehicle; a complete status alone does not prove every component is healthy.
- Misfire counter
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A count of events the computer classified as possible misfires.
Many systems infer misfire from crankshaft speed variation. Counter availability and interpretation vary. Compare the counts with operating conditions and manufacturer tests; a count does not identify a coil, injector or mechanical fault by itself.
- Live data
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Current values and calculated parameters reported by vehicle control modules.
Live data can include sensor readings, commands, calculated values and statuses. Update rates and coverage depend on the module and tool. A displayed command does not prove that a component moved or that a signal is accurate.
- Bidirectional control
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The ability to request supported component or system tests with a scan tool.
Coverage varies by vehicle, module and software. Commands can operate fans, pumps, valves or other equipment, so follow the test prerequisites and safety instructions. Confirm the physical response rather than trusting the command alone.
- Pending code
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A detected condition that has not yet met its confirmation rules.
Many emissions faults use more than one qualifying trip, but confirmation and warning-light rules vary. Pending records can help investigate intermittent faults. Save them with the rest of the diagnostic evidence.
- Limp mode
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A reduced-operation strategy used when a control system detects certain faults.
Depending on the fault, the vehicle may limit throttle, engine speed or transmission operation. This is not a guarantee that driving is safe. Use the warning messages, symptoms and owner instructions to decide whether to stop and obtain assistance.