P0503 Brake Failure but only on hot days

[AussieTimmeh]

My XC90 has started displaying a Brake Failure - Stop Safely message along with some warning lights, roughly a couple of minutes but only on hot days, roughly 30°C or hotter. The tripmeter is dashed out and the odometer stops incrementing. Cruise can be selected but will not engage. The brakes still work but I suspect ABS might not. I get a pending P0503 code in this mode.

Then the car will cycle between 3 different states: Brake Failure, no errors at all, and Engine Service Required with the check engine light on. The trip meter and odometer work fine when there are no errors or when only the check engine light is on. I get a current P0503 fault code in these other two modes, although sometimes it all goes away, I guess if the car hasn't experienced a hot day for a little while, or I clear it manually.

In all 3 states, the car seems to drive fine. I've read stories of similar failures with the tacho not working, the gearbox not shifting, the throttle not responsive, but I don't get any of that. The only thing that is consistently not working is being able to select cruise during any failure mode, but this starts working again if I clear the codes and power cycle the car.

On cooler days (less than 30°C, there is no problem and I wouldn't know there was an issue.

I've done a lot of googling and read in a variety of Volvo forums that it could be a speed sensor on the wheel, or perhaps a module like the ABS module or BCM. Other people suggested dry solder joints, some say pedal sensor. One thread suggested a plug into the CEM comes loose.

I'd like to know where the vehicle gets its speed signal from, because that seems to be working fine at all times (speedo), so why does the odometer stop working I wonder?

I swung by my local mechanic this morning and ran the G-Scan over the car. It came up with the following fault that seemed relevant: 1A5F Communication with the ABS control module. Signal missing.

I'm not sure if it's the ABS module not communicating with a single sensor, or the car not communicating with the ABS module. This site would indicate it is a problem with the module itself: https://www.dtcdecode.com/Volvo/CEM-1A5F

I wonder if the ABS module is having issues on hot days, or is it a sensor that the ABS module is trying to communicate with?

Any help would be appreciated!