Glossary
Quick answer
The terms that recur across every trouble code page, defined plainly. The three worth knowing before anything else are freeze frame, which is a snapshot of sensor values taken when the code set, fuel trim, which is the correction the computer is applying to fuelling, and readiness monitors, which are the self tests an emissions programme checks.
| Term | What it means |
|---|---|
| Bank 1 / Bank 2 | The side of the engine containing cylinder number one is bank 1, and on a V engine the opposite head is bank 2. An inline engine has only bank 1. Which physical side is which varies by manufacturer, so confirm it for the specific engine rather than assuming. |
| Bidirectional control | The ability of a scan tool to send a command to a module rather than only read from it, so it can cycle a fan, run a system seal test or cut out one cylinder. No read-only tool can emulate this at any price. |
| Closed loop | The operating state in which the computer adjusts fuelling based on oxygen sensor feedback. Before the sensors reach operating temperature the engine runs open loop on a preset map, and fuel trim readings taken then mean nothing. |
| Drive cycle | The specific sequence of cold start, idle, acceleration and steady cruise conditions a vehicle must complete for its readiness monitors to run. Procedures differ meaningfully between manufacturers and there is no universal version. |
| DTC | Diagnostic trouble code. The five character identifier, such as P0420, that names the condition a module observed. |
| EOBD | The European equivalent of OBD-II, required on petrol vehicles from 2001 and diesel vehicles from 2004. It uses the same code structure and connector. |
| EVAP | The evaporative emission control system, which captures fuel vapour from the tank and feeds it into the engine to be burnt rather than vented. Small leak codes here describe a hole roughly the size of a pinhead. |
| Freeze frame | A snapshot of every available sensor value recorded at the instant a code set. It is the single most useful piece of diagnostic evidence available, and clearing the code erases it. |
| Fuel trim | The correction the computer applies to fuelling, expressed as a percentage. Short term is the immediate correction and moves constantly; long term is the learned average. Positive means fuel is being added, so the mixture was lean. |
| I/M readiness | Inspection and maintenance readiness, the set of flags showing which emissions self tests have run to completion. An emissions programme reads these to confirm the computer has actually checked itself. |
| Limp mode | A protective state in which a module restricts operation, commonly by limiting engine power or gear selection, to prevent damage. It is the vehicle protecting itself rather than a fault in its own right. |
| MIL | Malfunction indicator lamp, the formal name for the check engine light. A steady lamp and a flashing lamp are different messages, not different intensities of the same one. |
| Mode 6 | The diagnostic mode that exposes the raw results of on-board monitor tests, including the measured value and the pass threshold. It shows a monitor drifting toward failure before a code ever sets. |
| Monitor | A self test the computer runs on one emissions subsystem, such as the catalyst monitor or the evaporative monitor. Each reports complete or incomplete. |
| OBD-I | The manufacturer-specific diagnostic systems used before OBD-II. No common connector, protocol or code list, so a generic scanner cannot communicate with one at all. |
| OBD-II | The standardised on-board diagnostic system required on all passenger cars and light trucks sold new in the United States from the 1996 model year, through a common 16 pin connector. |
| Open loop | The operating state in which the computer fuels from a preset map rather than from oxygen sensor feedback, typically when cold or at wide open throttle. |
| PCV | Positive crankcase ventilation, the system that draws crankcase vapours into the intake. A failed PCV valve or a split hose is a routine cause of lean codes. |
| Pending code | A fault the computer has seen once but not yet confirmed. Most emissions monitors require the fault on two consecutive drive cycles before the lamp comes on. A pending code is an early warning that is often still cheap to act on. |
| Permanent code | A code stored in a way that cannot be cleared by a scan tool. It clears only when the computer itself confirms the fault is gone across the required drive cycles, which is specifically designed to stop a code being cleared before a test. |
| PID | Parameter identifier, the standardised address for one live data value such as engine coolant temperature or short term fuel trim. |
| Rationality test | A check that compares one sensor against others to see whether its reading makes sense in context, rather than merely checking it is inside an electrical range. This is how a sensor that is lying plausibly gets caught. |
| Readiness monitor | See monitor. The flag showing whether a given emissions self test has completed since the last clear. |
| Stoichiometric | The air to fuel ratio at which combustion is theoretically complete, about 14.7 parts air to one part petrol by mass. Closed loop fuelling targets it. |
| Upstream / downstream sensor | The oxygen sensor before the catalytic converter is upstream and controls fuelling. The one after it is downstream and mostly judges how well the converter is working. Confusing the two is a common and expensive mistake. |
Common questions
What is the difference between a steady and a flashing check engine light?
They are different messages. A steady lamp reports a stored fault that needs attention. A flashing lamp reports an active misfire severe enough to send unburnt fuel into the exhaust, where it can destroy the catalytic converter within minutes. A flashing lamp means reduce load and stop driving, not drive more carefully.
What is the difference between upstream and downstream oxygen sensors?
The upstream sensor sits before the catalytic converter and its signal controls fuelling in closed loop. The downstream sensor sits after the converter and mostly judges how well the converter is working. Replacing the wrong one is a common and expensive mistake, particularly on catalyst efficiency codes.
What does bank 1 mean?
Bank 1 is the side of the engine containing cylinder number one. An inline engine has only bank 1. A V engine has bank 1 and bank 2, one for each cylinder head. Which physical side of the engine bay each occupies varies by manufacturer, so confirm it for the specific engine rather than assuming it is the driver side.