A recent, frustrating job involved tracing a fault code 86-01 “F-CAN communication malfunction” on a 2009 Honda Accord. The short story is that the fault was caused by the YAW rate sensor and the description for the fault code appears to mean “A fault on one of the components connected to the CAN wiring” which is very “general” and misleading.
Initially when the vehicle came in it only had code 86-01 “F-CAN communication malfunction” and a quick Google brought up a troubleshooting page which discusses checking the red and white wires for CAN communication between the ABS module and the ECU, and if they are good “substitute a known good ABS module”. This all suggests it’s all about the communication between the ABS module and the engine ECU.
An oscilloscope was used to check the CAN communication - after first measuring the resistance of the CAN wires at 60 ohms. A “logic” approach was to disconnect both the ABS pump and the ECU while scoping the CAN wiring - this turned the CAN pattern to mush until EITHER the ABS OR the ECU was plugged back in again, then the signal returned to ‘normal’. This proved that there had to be proper contact of the CAN wires at either of the modules.
A “good” used ABS pump was located and fitted and the code returned. Basically the code can be cleared and the ignition cycled as many times as you want and the code doesn’t return until you start the engine. Then one site suggested checking the CAN wiring to the instrument cluster which yielded nothing positive. The place that supplied the ABS pump loaned us the ECU to try as well but the fault just returned.
The next step was to check the live data, maybe that should have been the first step. This was partially informative and confusing at the same time. The steering angle sensor reading showed the correct angle when the steering was turned but underneath that was “Steering angle(CAN)” which showed in the red at 1170 degrees and never changed when the steering wheel was turned. Was this the fault and what was “Steering angle(CAN)”?
The other option was the YAW/Lateral sensor which showed as “normal” when the code was cleared but as soon as the vehicle was started it showed as “failed” and the zero reading changed to a random number. The YAW sensor was under the gear shift lever and the donor vehicle still had its YAW sensor. The “good” used YAW sensor was fitted and this actually fixed the issue of the reading failing when the car was started. BUT: The fault code 86-01 persisted.
The turning point was when someone suggested that maybe that was the whole problem in the first place and the ‘new’ pump had an issue of its own. At this point I remembered that when we fitted the donor ECU it brought up a dozen fault codes for the transmission, because it came from an automatic and this was a standard. There was a difference in the programming of the ABS modules and the ‘new’ pump had an issue with “A fault on one of the components connected to the CAN wiring”. Who knows what it was but fitting the original ABS pump fixed the problem!
The lesson learned was “Before changing components based on a fault code first check all live data” - keeping in mind the fact that live data isn’t always perfect and can still provide misleading information of its own [Steering angle (CAN)].