Bosch CX (BDU450, Kiox) 503 speed sensor fault - disappeared

[Kiwimike]

Went on a 20k canal path ride. At the start there was an intermittent 503 speed sensor error, that eventually became permanent. No change of magnet etc. The assist never stopped completely, just felt like Eco mode. Was able to pedal in the 11T cog and keep up with the group. Speed showing 0. So that was weird.

Was expecting to have to replace the sensor - just took it for a ride today and everything’s fine! Only change was removing the battery, driving for two hours home and reinstalling it.

I could see the 503 fault in the diagnostics (I’m a dealer) - and there was a battery disconnect fault. Both cleared and didn’t return.

I’m wondering if the battery wasn’t perfectly seated at the start of the canal ride - could that cause a 503 error somehow? Like I said, weird, but that’s all I can think of.

Ideas welcome.

 

[Stihldog]

I wonder if the r&r of the battery, and power connection, was enough to solve the issue. It’s happened to me before (Trek Rail with Kiox). An intermittent connection may cause that. I just powered through it on a ride. Found the problem later. The battery and cover was not seated properly.

The speed sensor on the rear triangle is a simple enclosed reed switch, activated by the magnetic on the rotor. A buildup of metal filing on the magnet can cause a fault. Alignment of the sensor also, or a bad connection or faulty sensor.

You probably know this stuff but that’s been my experience with a few bikes.
Good luck with this mystery.

 

[Bear-uk]

I had the error a few weeks ago and found out the plug had worked it's way out of the motor.
Pushed it back in and everything was ok.

 

[Kiwimike]

Aergh - after that ride, did two more of 35km each with no issues - now it's back again. did 20km of reds, then whilst riding along on a road, it threw 503 again.

Dropped the cover off, the sensor is still plugged in, with no sign of anything weird.

I'll fit another sensor, because that's the quickest / easiest next thing to do.

 

[Stihldog]

Aergh - after that ride, did two more of 35km each with no issues - now it's back again. did 20km of reds, then whilst riding along on a road, it threw 503 again.

Dropped the cover off, the sensor is still plugged in, with no sign of anything weird.

I'll fit another sensor, because that's the quickest / easiest next thing to do.

I know it sounds completely unrelated but a poor alignment of the battery cover may be causing intermittent electrical connections to the battery.

The battery sits on a bulkhead which holds the wiring harness. The bulkhead is slightly adjustable which can insure a positive connection between the wiring harness and battery.

Two days ago I diagnosed the same issue, on a friend’s bike. He had loss power, error codes, a rattle etc. I could see that the battery cover was poorly adjusted. I loosened and adjusted the bulkhead for him. Problem solved.

It doesn’t take much adjustment but you may have to drop the motor or skid plate to gain access to the adjustment bolts (very small). And sometimes it doesn’t take very much movement.

Don’t use dia-electric grease in this area. That could make problems worse. Besides a greasy mess could be created. If water gets in…it must get out. Or at least have a chance to evaporate.

 

[Damo888]

Aergh - after that ride, did two more of 35km each with no issues - now it's back again. did 20km of reds, then whilst riding along on a road, it threw 503 again.

Dropped the cover off, the sensor is still plugged in, with no sign of anything weird.

I'll fit another sensor, because that's the quickest / easiest next thing to do.

I had the same problem going into limp mode every now and then until it eventually stopped working and error code 523001 "speed sensoe is not working correctly"

Took it to a bike shop and they replaced the sensor cable and now working. They had it for 3 weeks and charged me $360 AUD.

I could of replaced it in an hour I think.