ABS light on intermittently - no error codes

Question

I had this problem ever since I bought my car 7-8 years ago. The ABS light turns on rather randomly. Sometimes it stays off for months. Sometimes it turns on and of multiple times per day. It's a Ford Focus 1.8 tdci from 2004, 100 HP.
I did check the following:

brake fluid level and cap checked (replaced with one from another car to see if the problem disappears, it didn't)
DTC error codes read with Forscan: no error codes in general, no error codes from the ABS module. The only place in the OBD scanner tool where I could find something remotely related to this problem was the "ABS warning indicator" which was consistent (on/off) with the warning light on the dashboard
reading ABS data while the vehicle is moving: all wheel sensors give the same speed reading in a straight line (with the ABS warning light on or off)
performed the Forscan OBD ABS troubleshoot test: the test performs ok and no errors are found
sometimes, after washing the car, the ABS light stays off for some time
I tested the car during winter: the ABS works even if the ABS warning light is on

I'm not sure what to do next in order to solve this problem. I have a few questions

Is it possible that I have a faulty sensor even if the speed read for each wheel is consistent with the speed of the vehicle? Can sensors be bad without giving error codes?
What are your suggestions regarding what I can try in order to isolate the problem?
Is it possible that this is a dashboard problem? I took it off and saw some bits of corrosion on some of the nodes on the electronic board.
If the ABS module is available in Forscan for the live-sensor data, that means that DTCs should also be available, if present? Update: I took one of the sensors off and managed to get an error code. Therefore, indeed, the ABS light stays on without any error code.

Thank you in advance.

Jupiter · Answer

Speed sensors are prone to be intermittent in failures. If they can easily be removed and cleaned without breaking them cleaning then might help. I believe this car has a pedal pressure sensor that is found by the brake light switch. I'm confused by the anti locks working during an illuminated ABS light. That shouldn't happen. I'm assuming your scanner can read ABS codes, but if the light turns on, there is a code stored. You should confirm which codes these are by verifying the scanner is able to read these. Or better yet find another scanner that can read ABS codes for this specific vehicle. This should be done before any other parts are replaced. The only thing I would highly suspect at this point without a code would be a bad brake fluid level sensor or the wiring or connectors to it. I know you replaced it with a used one, but that doesn't mean it's good.

Answered by Jupiter on August 17, 2021

Beni Bogosel · Answer

It turns out that this was an Instrument Cluster problem. After swapping the instrument cluster with a different one the ABS light stays off.
Therefore if the ABS lights (or other lights) come on without generating any error codes you might try to see if the problem is the Instrument Cluster itself.

Update: I tried restoring the old instrument cluster since the new one had the wrong mileage. It turns out that cleaning all the rust points using a contact spray did the job and the ABS light is off now.

ABS light on intermittently - no error codes

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