Cause
Metallic debris on the sensor tip
Ferrous debris collected on a magnetic speed or position sensor's tip, distorting the signal it produces. These sensors work by detecting a passing tooth, and swarf bridging the gap changes the magnetic circuit — so the signal weakens, gains noise, or drops out.
The debris is a symptom worth reading. Fine magnetic paste is normal wear collected over years; visible metal chips are not, and they mean something inside is wearing badly. So finding a sensor tip covered in swarf is a reason to inspect the assembly, not merely to clean the sensor and refit it.
The fault is intermittent and load-dependent, since debris shifts with vibration and temperature — a speed signal that drops out under load and reads correctly on the ramp is the classic. Wipe the tip and inspect what comes off before ordering a sensor, and check the air gap while it is out, since a sensor that has been disturbed and refitted at the wrong depth produces the same weak signal with a clean tip.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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