308 hdi fuel cap sensor fault

[willnz]

Have just purchased a 308 T7 HDi 2.0 5door sedan hatch. vin VF3**************[VIN obfuscated, can be read by forum staff]
Ran diagbox to check for any faults and came up with P2408 fuel cap sensor fault which is triggering dpf system error. Had a look under the wheel liner and inside the boot liner and there is no sign of any pickup sensor/wiring etc like I have previously found in 307, 407 etc cars I have owned, so am at a bit of a loss as to why this code is being thrown. I am worried as I know this lack of detection can prevent the dpf system from injecting into fuel tank. I aLso checked the wiring diagrams in SEDRE and found no trace of a circuit.
Am I missing something?

[Gibbo2286]

Are there magnets in the fuel filler cap? They can fall out. The sensor is usually close by under the rubber surrounding the neck.................if it has one.

[GiveMeABreak]

Whether you have the Fuel flap sensor depends on the engine management system fitted. Earlier vehicles had the Delphi DCM 3.4 engine management system and later 308 T7s had the Delphi DCM 3.5 version which did away with the sensor. Instead, it uses the engine ECU to calculate whether any fuel was added and the quantity where this is the case, to determine the amount of additive to inject.

If you have the earlier system with the fuel cap detect sensor:

(8) Contact
(9) Permanent magnet

The filler cap is fitted with a magnet and when the cap is closed, the magnet is opposite the contact.

As for the code:

Fault Code:	P2408
Description of Fault:	Additive system: Fuel tank cap sensor signal fault. Additive reservoir cap signal incoherent for 3 seconds.
Conditions for Fault to clear:	Ignition on
Downgrade Modes whilst Fault is active:	Deactivation of the forced regeneration of the particle emission filter
Symptoms:	-
Suspect Areas:	Additive adding

Diagram:

4320: filler cap presence switch

[willnz]

It has the Delphi DCM 3.4 ecu but there is no wiring/sensor (assumed to be a reed switch) attached to the filler ( I removed both the wheel well and boot covers to try to find the sensor/wiring), nor is there a magnet in the fuel cap I can detect by using small metal pins, that is why I am at a loss to explain the error code being thrown.

Ok, being the pack-rat that I am I have a 308 1.6 HDi that is an accident wreck (parts car for my other 1.6HDi). After reading your post above, I dug into it and found that it has a magnet cap and a cap sensor. The sensor and associated wiring is extremely well hidden on the filler neck (had to shove it down into the opening to even see the sensor), with a junction plug under the car. Removed it and checked the 2.0 car. Magnets confirmed missing for the cap and no sensor on the filler neck, but has the junction plug underneath..... swapped it all in and error code gone.. wtf?? Why did it not have this installed? Also, how has the car gone for this long (230000km) with this fault which supposedly shuts the dpf fluid down?

Anyway, many thanks Marc for the wiring diagram and pic of the sensor and magnet, it spurred me onto finding the bits on my parts car.

[GiveMeABreak]

Just to confirm that not having the plug detect sensor does not shut down the additive function. It is only used to advise the ECU that fuel may have been added. The engine ECU will still undertake fuel measurements and should still dose the fuel tank accordingly. It's not really these days needed as the fuel measurements can be used to determine any fuel added, so was probably eliminated so save costs.

The downgrade mode in the fault code table above is just referring to preventing forced regeneration, meaning using a diagnostic tool to force a regeneration operation. Normal regeneration should work fine.

[Paul-R]

Is "normal" regeneration just high temperature regeneration that doesn't need Eolys or does it include reduced temperature, Eolys assisted, regeneration?

[GiveMeABreak]

When I use the term 'Normal' I mean 'Passive' regeneration. Passive regeneration occurs at about 550 °C, the soot naturally burns off at this temperature in the filter.

When the filter is clogged and the engine is cold or warm, Assisted regeneration kicks in, which works at lower temperatures (450 °C), whereby the engine ECU puts an extra load on the engine through use of Glow Plugs, post injection and electric consumers to get it to this temperature more rapidly.

As Eolys is already present in the diesel (with a correctly functioning system) under normal working conditions, so assists with combustion in any case.

Patterns of driving and driving conditions are stored by the engine ECU, so it has a history and can determine the best strategy for regeneration, including failed previous attempts to regenerate, soot loading etc.