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Discussion Starter #1 (Edited)
Noob here. So I picked up a 91 Miata a month ago and haven't had time to work on it until now.

It had a CEL and I scanned it and received code 9 which is supposed to be a water thermistor malfunction or short circuit. So I picked up the part at autozone (Part number 8574-18-840) and replaced it, but even after reconnecting the battery the CEL still remains as a code 9.
The car also still idles high and at random times fluctuates without touching the gas like before.
Not sure what I should try next. I tried checking the wires and plug that goes into the water thermistor for damage but I'm also a noob and have no idea what I'm looking at.

Any ideas on what I should do now?

Also sorry in advanced if I messed up and somehow missed a valuable link, but I really did try and search before posting haha

Edit***
I also took a video of the CEL flashes and I think I may have missed a Code 10 as well(The CEL that lights up for a longer time after the 9 flashes)...I'd appreciate if someone would clarify such a dumb question.

Also, I don't know if it'd help but I took a video of the car idling and what happens when I step on the throttle...when I only give it a little throttle the engine chokes but it doesn't choke if I stomp on the pedal...
 

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If the plug was at fault, likely it would be damaged or it would be corroded, both of which should be pretty easy to tell. At this point, if it were me, I would use a continuity tester (or multimeter on that mode) to test for postive voltage at that plug (should be the blue and white wire), grounding the tester to a known ground. If you get nothing, there is likely a short in the positive lead to that sensor. If you do get voltage, then you may have an issue with the ground coming from that sensor.
 

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Discussion Starter #3
If the plug was at fault, likely it would be damaged or it would be corroded, both of which should be pretty easy to tell. At this point, if it were me, I would use a continuity tester (or multimeter on that mode) to test for postive voltage at that plug (should be the blue and white wire), grounding the tester to a known ground. If you get nothing, there is likely a short in the positive lead to that sensor. If you do get voltage, then you may have an issue with the ground coming from that sensor.
Thanks for the reply, I'll try and check out the voltage in those areas.
 

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You only have a code 9. That odd one denotes that the codes are finished displaying and restarting again. It makes it easy if you had multiple different codes, after it cycled through all of them, you would get a long flash like that, and it starts again.

The coolant temp sensor, has a direct effect on the fuel map, much like the airflow meter and the 02 sensor. If it reads incorrectly or not at all, it can cause some issues idling or running.
 

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Discussion Starter #5
Okay so this weekend I checked the wiring with a multimeter (or at least attempted to) and came up with a voltage of about 2.1 when probing the green plug while the key was turned to the on position. When I checked for resistance in the plug (not actually sure if I did that right, but I probed the plug with the red wire and probed the bolt/bracket at the back left of the engine with the black wire) and got a reading of 200k Ohms so I guess there's an issue with the ground. And while there I found this...


With high resistance, is this the troublesome ground I've been looking for that is causing my code 9? If so how do I go about fixing this? I have absolutely no experience with wiring or soldering or anything like that.
 

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Yeah, that's the ground for the sensor, but I can't remember if there should be 2 wires to it. I think mine only has one. I'm chasing down a problem with mine as well.
 

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Discussion Starter #7
Yeah, that's the ground for the sensor, but I can't remember if there should be 2 wires to it. I think mine only has one. I'm chasing down a problem with mine as well.
Any tips on how I should approach this? I'm sure it's a dumb question but I'm pretty inexperienced and don't know if a new ground wire has to be soldered in or something. Maybe hire someone else to replace this wire?
 

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Discussion Starter #8
Figured I should update this if someone else is having a similar problem. I winded up taking the car to a shop. Turns out the problem was due to both bad connectors and a badly corroded ECU (PO did not reinstall the ECU cover).

The mechanic winded up splicing in new connectors for the coolant temp sensor and airflow meter (the mechanic said the blink after the code 9 was in fact a code 10) and put in a new ECU. The connector leading to the ECU was also in bad shape (a few wires jammed in with no pins) and he did what he could with it, but recommended splicing in a new connector if I could find one.

All in all, the car idles at about 850 RPMs and runs/pulls fine now and has no codes. The repair took $540 bucks out of my pocket which was not ideal, but I think its a fair price considering the amount of labor hours my mechanic actually put into the car (or so he told me, but I believe him).
 
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