In warm and dry weather, the diesel engine will start even if one glow plug is defective and only the other plugs preheat. With glow plug failure there will usually be increased pollutant emission and possibly also knocking during start, however, the driver will not consciously take note of these signs, or will not know how to interpret them.

There will be a rude surprise once the weather becomes cold and clammy and the first night frost sets in: the "heat supply" of the diesel engine fails to function, and it will start difficulty and smoke at best – most probably, however, nothing will work at all.

Causes & Corrective Action of Glow Plug Failure

Below is a list of typical damage and the respective causes. In most cases, it will be possible to correct the fault with this diagnostics aid.

Folds & DentsGlow Plug Failure Heating Rod with Folds & Dents


  1. Operation at too high voltage, e.g. jump start.
  2. Too long power supply due to a stuck relay.
  3. Impermissible post-heating when engine is running.
  4. Use of a non post-heating glow plug.

Corrective action:

  1. Jump start only with 12 volt on-board power supply.
  2. Check preheating system.
  3. Replace glow time relay.
  4. Install post-heating glow plugs.



  1. Beginning of atomization too early.
  2. Coked or worn nozzles.
  3. Engine failure, e.g. because of piston.
  4. Jamming, valve breakage, etc.
  5. Dripping nozzles.
  6. Seized piston ring.

Corrective action:

  1. Set injection point accurately.
  2. Clean injection nozzles.
  3. Check jet.



  1. Atomization begins too early, and heating rod and heating coil are overheated during this; the heating coil becomes brittle and breaks.
  2. closed annular gap between plug housing and heating rod; as a consequence, too much heat is deflected from the heating rod, the regulating.
  3. oil remains cold and lets too much current flow to the heating coil, causing the latter to overheat.

Corrective action:

  1. Check injection system, set injection point accurately.
  2. When screwing in a glow plug, always comply with the tightening torque specified by the vehicle manufacturer.



  1. Torn off connecting bolt: The current connecting nut was tightened with excessive torque.

Corrective action:

  1. Tighten current connecting nut with torque wrench. Always observe specified tightening torque. Do not lubricate or grease the thread.



  1. Damaged hexagon: Use of incorrect tool; the plug is deformed and causes a short circuit from the housing to the round nut.

Corrective action:

  1. Tighten plug with suitable torque socket wrench. Strictly comply with the specified torque (refer to specifications of the car manufacturers).

Requirements of a Glow Plug

Glow Plug Glowing Inside EngineShort Heat Up Time

Glow plugs must provide a high temperature within an as short as possible time to assist with ignition, and it must maintain this temperature regardless of the basic conditions, or even adjust the temperature in dependence of the latter.

Small Space Requirement

In the past, the diesel engines of passenger cars mainly operated as direct injection engines with 2 valves and thus offered sufficient space for injection nozzles and glow plugs. In modern diesel engines with common rail or pump-nozzle injection system and 4-valve technology, however, the available space is very restricted. This means that the space required for the glow plug must be reduced to a minimum, resulting in a very thin and long shape. Today, BERU glow plugs with glow tube diameters that have been reduced to 3 mm are already in use.

Precise Adaptation To The Combustion Chamber

Ideally, the glow rod should be situated precisely at the edge of the mixture turbulence - however, it must still immerse sufficiently deep into the combustion chamber or the antechamber. Only then is it able to introduce the heat accurately. It may not protrude too far into the combustion chamber, as it would otherwise interfere with the preparation of the injected fuel and thus the carburetion of an ignitable fuel-air mixture. This would result in increased exhaust gas emissions.

Sufficient Glowing Volume

Apart from the glow plug, the injection system is of particular significance for the engine cold start. Only a system that has been optimized in terms of its injection point, quantity and carburetion in conjunction with the correct position and thermal rating of the glow plug will ensure good cold start performance. Even after the engine has been started, the glow plug may not be "blown cold" by the increased air movement in the combustion chamber. Very high air speeds are in particular present in ante- or turbulence chamber engines at the glow plug tip. In this environment, the plug will only work if it has sufficient reserves; i.e. if sufficient glowing volume is available so that heat can immediately be introduced in the cold blown zone.

The result: an environmentally-sound diesel quick start in 2-5 seconds.

Cold Starting

A cold start describes all start processes during that the engine and the media have not reached operating temperature. The lower the temperature, the less favorable are the conditions for a quick ignition and complete, environmentally friendly combustion.

Certain aids are used to assist during cold start and so that starting will not be unacceptably long or even impossible. These compensate for the poorer start conditions while initiating a well-timed and even ignition to ensure stable combustion.

The glow plug is one component that assists during cold start. It creates ideal ignition conditions for the injected fuel by electrically generated thermal energy that is taken in the combustion chamber. It is indispensable as cold start aid for engines with a divided combustion chamber to ensure that these can start even in the frequently occurring temperature range of 10–30 °C.

As the start quality deteriorates considerably below freezing point, the glow plug is also used as cold start aid for direct-injection diesel engines. Check out our range of Glow Plugs