The problem with CEM damaged by water intrusion in 2005/2006-models

All P2 Volvos from the years 2005 and early 2006 were created with a weakness that could allow water to enter the CEM computer inside the car. The CEM is the main computer of the car and its data are encrypted. You cannot switch the CEM from another car and it's very expensive to replace. So, what is the reason for this problem and how to prevent it?

Volvo issued the following service bulletin describing the problem with water ingress:

MODEL: All P2X (S60 S80 V70 XC70 XC90)
YEAR: 2005-2006 (structure week 200425-200540)
CHASSIS:
XC90 FC1 134000-256551
V70 FC1 459000-555060
S60 FC2 425000-522407
S80 FC1 390000-435423
XC70 FC1 173000-220845
V70 FC2 459000-554206
(FC = Factory code) 11TH position of the VIN number

Description:

Various electrical function and communication symptoms may occur in M/Y 2005-2006 vehicles caused by wet corrosion in the CEM (Central Electrical Module) connector. The grommet that seals the harness entrance to the Plenum box may not seal properly if it has been incorrectly assembled.

This may cause a number of different functionality symptoms and DTC's to be set. Examples include but not limited to: Headlight on, even if ignition key is taken out of ignition lock. Warning messages in DIM (Driver's Information Module) display. Warning lamps lit up. Brake lights always on. No start condition.

The cause of the problem

This plenum box has poor quality. It has several weak spots where water can go inside. In addition some of the cars has been incorrectly assembled (butyl seal missing, bad gaskets). A great number of people with these problems reported that the car was parked in a hill during a rainstorm (making the water flow around the plenum box).

However, this problem is not seen on cars before 2005. So what happened? Which change created this problem? Could it be that Volvo replaced the plenum box with another type? No, look at these photos from an early and late model:

2002 Volvo S80

2005 Volvo XC70

They look exactly the same. So this was not the change in 2005. But if we open the plenum box, we find the answer:

2002 Volvo S80 

2005 Volvo XC70

When you look inside the 2002, all you see are some cables and the floor inside the car. But when you look inside the 2005 you see nothing but the CEM computer. In 2005 the CEM was redesigned and moved right up to the opening. So when water enters it has no place else to go than straight into the CEM.

You can even see from the photos that the 2002 plenum box is only secured with 3 bolts, while the 2005 uses 4 bolts. I guess the plenum box was badly designed all along, but the problems did not appear before they redesigned the CEM, making it more prone to water intrusion.

How to prevent this from happening?

The plenum box is so large and is designed in such a way that it is impossible to use butyl sealant around the box. You could remove the box and afterwards seal it to the car. But that will make things quite hard when you later have to remove the box.

Volvo completely redesigned the plenum box and started using the new one in week 40 in 2005. This new box has a much better design and a thicker gasket. Volvo even made an inexpensive conversion kit for the 2005-2006 cars with the old design, and this is the best solution to this potential problem. The conversion kit has the following Volvo part numbers:

Plenum lid: P/N 30728860

Service grommet: P/N 30775689

Plus you also need a couple of plastic strips

This is what the new plenum box and grommet looks like after installing the conversion kit. A much better design for keeping the water out. And the new box is significantly smaller. You can even put butyl sealant around the box if you want to:

The job replacing it is done in a couple of hours. There are 3 cable harnesses. The two connected at the top are easy. But there is also a third harness going down to several connectors in the pedal area. Some of the cable clips can be a bit challenging to reach. But it is certainly doable.

Should I upgrade if I have a 2005 or newer, with the old design?

I would definitely say yes! You could drive for many years with no problem. But one day you are forced to park the car in a hill during a rainstorm, and suddenly the problem appears. These cars are now 15 years old or more. The gaskets could start to fail after so many years. So why take the chance? It is inexpensive to upgrade the plenum box. But if water gets into the CEM, that is very expensive!