Glossary
A warning on the dashboard rarely comes with an explanation of the terminology. This glossary explains the terms you will encounter in vehicle software, control units, diagnostics, emissions and key systems—in plain language, without promises.
Glossary
Key terms covering engine software, calibration maps and performance tuning.
A modification to the engine control unit software. The file stored in the ECU is read, adjusted and written back. Despite the name, this no longer involves replacing a physical chip. What can sensibly be achieved depends on the engine, hardware, vehicle condition and transmission.
Software tuning with standard hardware, without modifications to the exhaust, intercooler or intake. The term is not standardised: what matters is the equipment actually fitted to the vehicle and its condition.
A calibration designed around hardware changes, such as a downpipe, a larger intercooler or a modified intake. Without the corresponding hardware, calling a tune Stage 2 is not meaningful.
A table of values in a control unit that determines its response at operating points such as engine speed and load—for example, boost pressure, injection quantity or ignition timing. Software tuning involves making targeted changes to these maps.
The original file read from a control unit before any changes are made. It provides the basis for restoring the factory calibration and for comparison if anything needs investigating after the work.
A calculated value used by a control unit to check data consistency. If the checksum no longer matches after a file is edited, the unit may reject the file or enter a fault state.
An adjustment to the maximum speed set in the software. Technical feasibility does not automatically make it lawful or advisable: tyre ratings, suspension, brakes and vehicle approval must all be suitable.
Glossary
What control units do, how their data is read and what fault codes mean.
The computer that controls the engine. It processes sensor data and manages functions including injection, ignition, boost pressure and exhaust aftertreatment. Familiar families include Bosch EDC17 for diesel and MED17 for petrol engines.
The computer that controls the automatic transmission. It determines shift points, shift pressure and the engine torque permitted by the transmission. When performance is changed, the ECU and TCU calibrations must work together.
The vehicle’s standardised diagnostic system. The OBD socket provides access to fault memory, live data and system status. OBD shows what the vehicle has detected; it does not replace an assessment of the underlying cause.
A standardised code, such as P0401 or P2002, used by a control unit to record a detected condition. It identifies the symptom and the area involved, but not necessarily the failed component.
A snapshot of readings recorded when a fault is stored, such as engine speed, load and temperature. It shows the conditions in which the fault occurred and is often more useful for tracing the cause than the code alone.
Ways of accessing a control unit outside the usual OBD connection. Bench access powers and communicates directly with the removed unit; boot access also puts its processor into a special mode. Both require more work and are used when OBD access is insufficient.
Enabling read and write access to a locked control unit so its software version and tuning options can be assessed. Whether and how this is possible depends on the ECU type, software version and protection mechanisms.
Transferring a control unit’s data to a matching replacement so it can operate in the same vehicle. This requires compatible hardware and software, along with readable data from the original unit.
A protective mode in which the control unit limits power or functions because it considers an operating condition unsafe. Limp mode is a response, not a diagnosis; the fault memory provides information for investigating the cause.
Changing a control unit’s configuration so the vehicle recognises its fitted equipment or knows how a function should operate. Coding does not increase engine performance and is not a substitute for a repair.
Glossary
DPF, EGR, AdBlue, SCR and NOx systems, and their common fault symptoms.
A filter that traps soot from the exhaust and burns it off when temperatures are high enough. Repeatedly interrupted regeneration increases soot loading and can block the filter—often as a consequence of another fault, rather than the original cause.
The process of burning soot out of the particulate filter at high exhaust temperatures. Short journeys, frequent interruptions and sensor faults can prevent regeneration from completing.
Exhaust gas recirculation returns some exhaust gas to the intake to reduce nitrogen oxides. Dirty or sticking valves, leaking coolers and sensor faults can cause characteristic fault codes and performance problems.
A urea solution injected upstream of the SCR catalyst to reduce nitrogen oxides. If the vehicle detects a system fault, it will usually display a remaining-distance countdown before restarting is prevented.
The catalyst in which AdBlue helps convert nitrogen oxides into nitrogen and water. Fault symptoms often involve dosing, temperatures, sensors or overall system efficiency.
A sensor that measures nitrogen oxides in the exhaust and tells the control unit how effectively the aftertreatment is working. Its position in the hot exhaust stream makes it a frequent suspect, but the readings must still be checked for plausibility.
The warning light indicating that the engine control unit has stored an engine- or emissions-related fault. A steady light indicates a stored fault; flashing can indicate a condition that may lead to further damage.
Glossary
Abbreviations specific to Mercedes, BMW or the Volkswagen Group.
An electrohydraulic braking system fitted to certain Mercedes models, including the W211, W219 and R230. Warnings such as “Bremse Werkstatt” (brakes: visit workshop), or codes such as C249F and C2498, are safety-relevant and should be checked before driving further.
The electronic steering lock in Mercedes vehicles, forming part of the start authorisation chain. One documented symptom is a key that turns while the steering lock does not respond and the ignition and warning lights remain off. That symptom alone does not confirm a steering-lock fault.
The electronic ignition switch in Mercedes vehicles. It checks the key, participates in start authorisation and communicates with the engine control unit and steering lock. Faults can appear as a no-start condition and need to be distinguished through diagnosis.
BMW control units for vehicle access, start authorisation and body electronics, called CAS, FEM or BDC depending on the model series. Their functions include key recognition and immobiliser management.
A BMW control unit for lighting and body functions. A known failure pattern is loss of communication after a voltage disturbance, with lighting, electric windows or indicators no longer working.
Glossary
Immobilisers, key systems and start authorisation.
An electronic security system that permits the engine to start only with a matching, recognised key. The key, ignition electronics and engine control unit exchange data; if one part does not match, the vehicle will not start.
The small chip in a car key that identifies itself to the vehicle. It is powered by the reader antenna’s field rather than its own battery, which is why the emergency start position often still works with a flat key battery.
A system that detects a nearby key and allows the vehicle to unlock and start without inserting it. Faults often involve antennas, receiver modules, the convenience control unit or the key battery.
The vehicle’s unique 17-character identification number. It helps identify equipment, fitted control units and part numbers, which is why it is needed for key, coding and control-unit work.
Next step
Include the make, model, year of manufacture, engine and recorded fault codes so we can assess your enquiry from the outset.
Next step
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