The CLK55’s C1401 – A7/3m1 high-pressure and return pump fault lit up the ABS/ESP/BAS warnings and sent me looking for the cause.

If I resided in another state I would ignore the ABS/ESP/BAS malfunction notices on the dashboard, since I normally switch ESP off. But in California the car will not pass DMV checkup. With the clock ticking down to tag expiration, I decided to address it.

When I first read the “C-1401 – A7/3m1 (high-pressure and return pump)” on my scanner I assumed and feared that the pump and/or its black box would need to be sent out for a rebuild. On further investigation, I got lucky with a much simpler and cheaper fix.

First I checked the relay and the 40 amp fuse in the driver side fuse panel, and they were fine. Then I applied 12V power to the pump side of 40 amp fuse socket and discovered that the pump was fine, it hummed smoothly.

I checked for 12V at the ABS relay, with the key on, but I was seeing 0 volts. This led me to suspect the K40/4 relay module on the passenger side… it’s job is to supply clean 12V power to the ABS/ESP pump and control unit, via internal soldered relays. It is a known weak point.

The only somewhat difficult task in the whole swap process was disconnecting those 10 plugs in the K40/4 module. The large ones have two little levers on top that you squeeze towards each other. The small ones were a little tricky, there is a single tab to release each one and the plastic plugs can be a little brittle after years of use.

if you take plenty of photos showing where each plug goes, installation will be fast and easy. I reconnected the battery, plugged in my Youcanic UCAN C2 scannner and cleared codes. Final step was starting the car and turning the steering wheel lock to lock and then back to center.

Instead of a $400 rebuild of the ABS pump and/or its control, I escaped with a used $70 eBay K40/4 relay module. The module I purchased from Recycl BMWNBENZ looked new and included the fuses and the relay.