Eolys fluid: what it is and why it matters
If you drive a Peugeot, Citroen, DS or certain Vauxhall and Ford diesel models and your DPF warning light has come on, Eolys fluid is one of the first things worth checking. It is a component of your DPF system that most drivers have never heard of, but when it runs out, the consequences for your filter can be serious.
What is Eolys fluid?
Eolys is a cerium-based fuel additive used in certain diesel vehicles, primarily those built on PSA and Stellantis platforms. It is stored in a small separate tank, usually located near the fuel tank, and is automatically injected into the fuel in tiny quantities every time you fill up.
The additive works by lowering the temperature at which soot ignites inside the DPF. In a standard DPF system, regeneration requires exhaust temperatures above roughly 550-600°C, which typically only happens on sustained motorway driving. With Eolys in the fuel mix, the combustion temperature of the trapped soot drops considerably, allowing the filter to clean itself during shorter or lower-speed journeys, a process called passive regeneration.
What happens when the Eolys tank runs out
When the Eolys reservoir is empty, the fuel additive stops being dosed. The DPF can still attempt to regenerate, but it now requires higher exhaust temperatures to do so. For drivers who rarely take longer runs, this means regeneration either happens less often or fails to complete. Soot accumulates in the filter faster than it can be cleared, and warning lights follow.
In many affected vehicles the sequence looks like this: Eolys warning light appears first (on some models it may only appear as a DPF or service warning), then DPF fault codes are stored, then limp mode is triggered as the blockage worsens. At that point the filter may already need cleaning even after the Eolys is replenished, because the soot that built up while the fluid was absent has not been cleared.
Which vehicles use Eolys fluid?
Eolys is most commonly found in:
If you are unsure whether your vehicle uses Eolys, a VIN check or a call to us will confirm it quickly.
What to do if your Eolys is low or empty
The Eolys fluid itself must be topped up by a dealer or automotive specialist, it requires a specific fill procedure and the correct cerium-based fluid for your vehicle. It is not something that can be refilled at home with an off-the-shelf product.
Once the fluid is replenished, if the DPF has already accumulated a substantial amount of soot during the period without proper regeneration, it will need cleaning. That is where we come in. Our mobile chemical flush breaks down the soot deposits that have built up in the filter, clearing the blockage and allowing normal regen to resume once the Eolys system is working again.
Our service costs £275 fixed, covers all of Greater Manchester, and we come to you, no recovery truck, no waiting around at a garage.
Call us on 0161 694 7331 to find out whether a DPF clean is the next step for your vehicle.