Mine went on the blink last week. When I turn the ign. key to p2 the TPMS light goes off and then on again. A few seconds later the SLS inactive mssg comes on. Last week I drove off and after a few miles it went off and on Sunday it was fine. On Monday it refused to work at all and the left front starts to sag. (I haven't replaced that bag and that is why it shows there first I guess)

All fuses are fine. I tested the pump by itself and it works. The relay in the fuse box works when tested out of the car. I activated the relay that is mounted to the side of the battery with 12V and the pump comes on fine. Today I took the panel off the passenger side to check the valve block.

UPDATE
There are three connectors on the valve block. Clearly the front one controls the front air supply, the rear one controls the rear axle. The front and rear valves work fine (the top pin in each connector is gnd, the other two are for connectng the individual bags to the systems pressure). I now think the middle connector activates a valve to allow air from the struts/bags to flow back to the compressor via the main feed line in order to release air from the bags. I can see no other point to this valve in the center of the valve block.

Before I have the codes read at the dealer I am checking the level sensor output on each suspension corner. I found a post from CivDiv outlining how to check the sensors and I checked the right rear and right front sensor which are ok. It is fiddly but can be doneby taking the sensor of the frame and realsing the arm.

If the other sensors check out ok then I must conclude the fault is the EHC behind the glove box. What I do not understand is why the TPMS light comes on?

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.