Preheating unit: role, location and price

The preheating unit is part of diesel vehicles. Thus, it is part of the injection system and works with glow plugs to ensure good combustion of the air-fuel mixture. In this article, we explain in detail the role of the preheater unit, where to find it on your vehicle, what its symptoms are, when it failed, and what its purchase price is!

🚘 What is the role of the preheating unit?

Also known as preheating relay, the preheating unit allows, as the name suggests, heat the air present in combustion chambers... In addition, he is responsible for lighting preheat indicator is present on the dashboard of your car. Thus, it will control the preheating duration according to the engine temperature.

Depending on the type of injection of the vehicle, its operation will be different. Indeed, your engine may have a direct or indirect injection system and this will affect the role of the preheater as follows:

1. Diesel engine with indirect injection : This mainly applies to diesel vehicles manufactured before 2003. To start the engine, fuel is injected into the prechamber where it is ignited and then attached to the cylinder's combustion chamber. The preheating unit will be connected to a glow plug on each cylinder to raise the temperature of the air entering the latter, this is called the preheating phase;
2. Direct injection diesel engine : also called HDI engine, fuel is injected directly into the combustion chamber. Thus, the preheating unit no longer performs a preheating phase, but operates with each of the spark plugs in a post-heating phase. Thus, this allows, first of all, to limit the emissions of pollutants and significant noise during combustion.

🔍 Where is the preheating unit?

Your car's pre-heater box will have significantly different location depending on the model and make of your vehicle. Typically, it is located in engine compartment so that below sweeping so that next to the fuse box your car. Indeed, the fuse dedicated to the preheating unit is present in the fuse box, so it may be close to the latter.

It can often be found near engine glow plugs. However, if you find it difficult to locate its location on your vehicle, you can use two different methods. First, consult service book your car, where you can find a detailed diagram of all the components in the engine compartment.

The second method is to enter the model, year and model of your car on various internet sites in order to access an annotated diagram of its parts and, in particular, the preheating unit.

⚠️ What are the symptoms of HS glow plug box?

Your car's heater box may be damaged. If so, there are many signs to help you know about it. Thus, you may have the following symptoms:

• The preheat indicator is on. : if it flashes or is on continuously, there is no doubt that there is a malfunction in the preheating unit;
• Le engine warning light lights up on the dashboard : Running it indicates that a diagnosis is necessary because the engine part is no longer working properly. This malfunction may concern the preheating unit;
• The car does not start : you will have to turn on the ignition several times before your car can start correctly;
• It is impossible to start the car : If the preheating unit is broken, you will no longer be able to travel in your car.

Failure of the preheater box is relatively rare. In fact, glow plugs may be more likely to cause this type of manifestation.

💰 How much does the preheater unit cost?

A glow plug is more expensive than a glow plug relay because the latest technology is used for direct injection engines. Typically required from 120 € and 200 € for the preheating unit and between 50 € and 70 € for relays.

If it is replaced by a professional in a workshop, labor costs will need to be added.

The preheating unit ensures the combustion of air and fuel in a diesel engine, especially with glow plugs... To prevent the car from starting, pay attention to the serviceability of your preheating unit. As soon as faults appear, contact a mechanic for a thorough diagnosis!