Inverted crank signal

[Thomson2372]

Hi,
New to oscilloscope diagnostics so looking for some help. Have a mk2 mx-5 with intermittent no start. Cranks strong. No spark when not starting. Found that the crankshaft signal is inverted when it wont start. Seems to pull down from 5v instead of up (when its running it pulls up).
Anyone have any advice on this or seen similar?

[liviu2004]

Oem sensor or aftermarket?
Tone ring might be magnetized.