What to record and when to stop driving
Vehicles with a combustion engine. Reduced performance of an electric drive or a high-voltage system requires a separate assessment.
Reduced pulling power, a sudden restriction in performance and uneven acceleration can have different causes. First assess the vehicle's behaviour and any accompanying messages. Do not test the fault by accelerating sharply.
If the vehicle judders or vibrates strongly and loses power at the same time, stop safely and contact professional assistance. Whether you can continue is not determined solely by the colour of a warning light.
How the change in performance occurred
| What you have noticed | What to report or distinguish |
|---|---|
| Reduced pulling power with otherwise smooth running | Whether the change developed suddenly or gradually, and under what load. |
| Juddering or uneven running | Whether it also happens while stationary and which messages accompany it. |
| Engine speed rises but the vehicle does not accelerate as usual | This description may also direct the checks towards the transmission of drive. |
| Normal power temporarily returns after a restart | Report the original message as well; a return to normal behaviour does not confirm that the fault has been repaired. |
When to stop driving
If a red engine oil pressure or engine cooling warning appears, stop safely and switch off the engine. Decide on the next steps according to the exact message and the vehicle's handbook. Do not open a hot cooling system.
If the vehicle cannot be controlled safely or cannot continue safely in the traffic conditions, arrange professional assistance. Do not use repeated restarts to continue demanding driving with an unresolved fault.
Neither the warning light nor limp mode identifies the faulty part
Drivers usually use the term ‘limp mode’ to describe restricted power or drivetrain functions. The term does not identify which component is faulty or confirm that it is safe to drive on.
On a diesel engine, a glow plug light that stays on or flashes while the engine is running may indicate an engine management fault. It does not automatically mean the glow plugs need replacing.
Why not automatically try to regenerate the filter
A loss of power cannot simply be attributed to a blocked particulate filter. A drive to support regeneration is appropriate only when the corresponding warning is displayed and the conditions in that vehicle's handbook are met.
If the engine runs markedly unevenly, other warnings appear at the same time or its condition is unclear, do not try to clear the fault by driving at high engine speeds. Seek advice on the next steps first.
What to record and what the workshop should check
Report whether the engine was cold or warm, when the problem began and whether refuelling, servicing or another intervention preceded it. Note the exact message. If you have a diagnostic report, keep it before any fault records are cleared.
Operating values are compared and related parts checked according to the particular engine and the findings. For an intermittent problem, it may be important to capture its condition when it occurs. The checks are determined by the results, not by a part selected in advance.
Related advice and services: Diagnostics · The car intermittently loses power · The engine misfires and shakes after starting.