That's just the connector. The actual sensor is located on the bottom of the coolant bottle.

 

This is true but it still doesn't affect the car starting or operating.

 

100% correct.

 

You're confusing things. The sensor in the bottle ONLY measures the level, it has no influence on fuel mixture. The ECU does not look at its signal.

The coolant temperature sensor is a different thing, there is no light for it failing. There is a code, but it is not P0420 and that sensor failing usually causes immediate limp mode, which you would have noticed.

Not being able to restart while warm is a sign of low compression.

What are you seeing in OBD that leads you to believe that it is locked hot? What is the OBD coolant temperature reading?
If that was locked hot, your engine temperature gauge would always show hot, even when the car is cold. Is that the case?

Please provide the info you're seeing from the car, not your interpretation of it. It'll help us get to the bottom of this.

 

This p0420 reading kn my ob2 though says its possible its and ect

 

Car just started up and is warming up now, it was a little rough of a start and it reved up to about 2k rpm and slowly worked its way down,, its been about 5mins and the temp guage still has yet to move

 

So when you get the car up to temperature. Turn it off and try to restart it. The codes and sensors you are worried about wont affect operation of the engine. If it doesnt start hot you could be looking at serious engine issues. Just do the compression test and the block test and get back to us.

 

So I just drove around the block and then parked it and shut it off,, first time starting it back up it took 3 seconds, and now it starts up perfectly every time after that. Ive turned it off and on about 5 times now and it starts right up with no stall at all and idles perfect

 

It's possible the coolant temp sensor is responsible on the average car, but on the RX8 the car goes into limp mode if the computer detects an ECT fault. So the symptoms don't fit.

It is far FAR FAR FAR FAR more likely your cat is in bad health. Not restarting easily when warm also supports this, because cat death is followed shortly by engine compression loss.

Think about it, the code literally means 'the cat is not doing its job'. There might be ways the coolant temp signal could influence it, but it's a stretch. The first, easiest conclusion is that the cat is not doing its job because it's dead.

That's a good sign. Let's get those plugs out and see what's going on inside.

 

Thank you loki!,, I am in the position where I am hoping it is just a bad cat 😩,, that said though, to blow through oem cat and an aftermarket cat in 87k miles seems hardcore

 

So then bad ignition coils if compression test is ok

 

Is it also possible in any way that its an evap system leak

 

The aftermarket cat never had a chance, most aren't designed for rotary EGTs. The OEM cat did but if the previous owner let it run on failed coils, this is what happens. Number one killer of RX8s.

Ignition coil replacement is basically preventative maintenance on these cars. Stock coils, especially early versions, live 20-25k miles reliable. After that they're on borrowed time. Aftermarket solutions based on GM coils don't have this problem.

Different set of codes and symptoms. Do you also have EVAP codes?
If you want to investigate this further, now that you car idles warmed up, see what the OBD long-term and short-term fuel trims and AFR are at idle. Should be near 0.

 

Short term spiked once at 3.2% for one second after watching it for a couple minutes, and

long term is a consistant 7

 

7% long term is a bit weird, but within reasonable range. You might have a tiny vacuum leak but nothing that would upset the car. If it was in the 20's I'd worry more.

 

The on board monitor shows a lot of o2 sensor rich and lean readings,, then it also has this and this is what made me question evap,,, havent gotten any evap codes, but like I said this shows up in the monitor

 

Yeah those numbers are consistant as well, pretty sure it stays about that way throughout driving

 

Short term also bumps to lots of negative % sometimes is that normal?

 

Compression is likely not the issue. Loki had great suggestions so I'm sure you'll get it up and running perfect.

 

Short-term can bounce around, that's pretty normal. Positive and negative is OK.
The EVAP leak mention there sounds consistent with the slightly high long-term trim, but as you see it says small leak. Fuel trim exists to compensate for stuff like that, so while it's a thing, it's not hurting your car at the moment.

O2 will show rich and lean depending on what the car is doing. When you're off the throttle coasting, no fuel is injected, so it reads lean. There are various transient enrichments for throttle tip-in, etc. but those are all transient. Mixture is never stable, this isn't an airplane
Also the front O2 is a wideband which reads actual AFR, while the rear (cat) O2 basically only knows rich and lean, but not how much. It's supposed to bounce between the two. If the computer notices that it doesn't spend enough time in lean, it concludes that the catalytic converter isn't cleaning the exhaust and you get a P0420. There's a bit more to the self-diagnostics, but that's the gist of it.

 

Yeah,, shouldnt the sensors be trying to stabilize the mixture as best as possie tho, to keep it at around midrange,, when driving and watching the o2 sensor it bounces from lean to rich but never tries to regulate

 

No, there is no single best mixture. When you're at full throttle you want enrichment, when you're off the gas you want no fuel at all, and everything in between. Mixture is a function of load, rpm and a few other factors. All fuel-injected cars work like this.

If you're cruising on level ground and constant speed, the front O2 should show relatively stable mixture, the rear O2 may still bounce around because it's imprecise.

 

Just got back in my friends car with a swivel extention and spark plug 13/16 deep socket and am waiting for the enging to cool down to get the spark plugs off. Ive fired it up multiple times and its started right up, I also let it idle for about 10 minutes and it had no issues when going back to check, after I shut it off I tried to start it again just to check and it started right up, maybe a half to a second of a hiccup. But ye, now its cooling down

 

I would plan on at least trying to get the cat off to have a look inside of it when you're working on it

 

This is one of the leading plugs, any diagnosis? Just old plugs? Possibly coils?