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Discussion Starter · #1 ·
Ok so I have a 2004 Mazda 2 and I’m having a fair bit of trouble with the ABS unit or rather these error codes received from FORScan:

U0121 - FF - Lost communication from ABS Module
U0121 - C - Missing CAN message from the ABS
U1900 - FF - Missing the CAN message from PCM

First off the car would not start. Clearing the error codes allowed me to start the car however the codes remained and the speedo doesn’t work, the handbrake light is constantly on, and the check engine and ABS lights remain illuminated. So the car is mostly drivable although sometimes it will decide not to start again and requires an ECU reset and the codes cleared.

I’ve been trying to find information about this and can’t find a whole lot of useful information and most of the posts I’ve read around with similar questions have gone unanswered. I’ve tried checking and cleaning the wheel speed sensors, checking the battery, replacing the ABS unit, resetting ECU, clearing codes and I’ve tried testing the terminals for ground outlined in the Mazda 3 document below. Perhaps the Mazda 3 layout is different to the Mazda 2, if that is the case can someone link me to the relevant one? According to the diagram I should have continuity across the wheel speed sensor terminals to the ground terminal but there is none on any of the points. Here is a picture of the ABS connector for reference. I have continuity from the bottom left terminal to ground (0.5ohms) so I believe I have it the right way around.

Where do I go from here? Someone please help.

Mazda 3 Service Manual - ABS HU/CM Inspection - Sensors, Switches, Relays


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