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Discussion Starter · #1 · (Edited)
Thought I'd share in case it was useful to anyone else. Today I was driving, stopped at a light, then started and noticed the car wasn't shifting gears and the speedometer was dead. I managed to get back home (probably in first gear) with a few other weird things happening ("wrench" light came on, fuel light came on). Once the speedometer briefly kicked back on and I felt the transmission shift, but it went back to being broken pretty quick.

I have an ELM327 scanner and pulled a P0720 code indicating an OSS circuit malfunction. Did some reading and found that it would likely be the output transmission speed sensor or wiring related to that. I was able to find the sensor pretty quick, unplug the wiring harness and see that it looked ok. Before just replacing the sensor I wanted a better idea that it was bad. I stumbled across FORScan and was able to make a couple plots driving around. Sure enough the OSS output was blank with a few faults indicated.
Rectangle Slope Font Line Plot


So I picked up a Duralast SU8818 from autozone and replaced the sensor (surprisingly easy job, although be aware that a bit of transmission fluid will leak when you pull the old sensor out). Turned the car back on, cleared codes, took it for a test drive and all looked good. I even captured another plot to confirm the new sensor was working correctly.
Rectangle Slope Font Pattern Parallel

I was curious about why the old sensor failed. I think these are just hall effect sensors so they shouldn't have moving parts or anything mechanical that breaks. One thing I did notice was that there was a lot of shiny buildup on the sensor.
Finger Nail Thumb Household hardware Auto part

My best guess is that these are metal fragments that accumulated over time and probably interfered with the sensor. I guess that's why there's a magnet in the transmission fluid pan to try and collect these(?). All said and done, the part is pretty cheap so replacing it after driving the car 120k miles doesn't seem that bad. I'm wondering though if I should replace my transmission fluid...
 

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2004 ZX3 SVT EE 173k miles. 2004 ZX3 SVT 194k miles
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Most likely the failure is from heat cycles. The expansion and contraction eventually breaks an internal wire and that's all folks. The sensor is designed to "see" a gap when a relatively large metallic mass with a gap is rotating in front of it. The metallic sheen wouldn't have any effect. Great write up.
 
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