I am not seeing normal O2 sensor voltages. The front sensor seems to be flatlined at 0, and the rear is fluctuating (See images - one is the RX8, the other is a normal readout from a Miata under similar conditions for comparison). I also just had a P0420 code show up.

The car is a 2004 MT with 61,XXXmi. The cat is a BHR mid pipe. The rest of the car, including ignition and oil, is in good health.

Before I start throwing parts at this, would you suspect something other than the front O2 sensor?

Finally, since I've been lurking and reading rather than posting, thanks for all of the help thus far!

 

The Miata's front O2 sensor is a narrowband sensor.

The RX-8's front O2 sensor is a wideband sensor.

Comparing voltages between them isn't valuable.


0v is full lean, and will typically happen when you are completely off throttle while still in gear, so hitting that is normal.

I don't really understand what issue you think you have though. Do you have a mileage problem? power problem?

 

CEL P0420 and corresponding mileage drop are the issues.

 

P0420 is Catalytic efficiency below threshhold. This is because you have the midpipe. Sorry, I didn't realize you thought that was a problem. If you don't want that code, put in a cat or flash the ECU to block it.

Mileage drop could be a whole host of possibilities, including:
- Old spark plugs (every 8-20k)
- Old/failing Ignition coils (every 20-30k)
- Old/failing Plug wires (every 30-60k)
- Clogged air filter
- Vacuum leak
- One or more intake valve acting up
- Clogged cat (not your issue here, since you don'th ave one)
- Engine compression loss
- Excessive engine carbon
- e-shaft sensor fouled
- e-shaft sensor profile inaccurate
- MAF fouled
- Fuel trims inaccurate
- grounding points corroded
- brake pads dragging
- tire pressures too low
- Front O2 failing
- Rear O2 failing (if present)

And probably a few others.

Nothing seems wrong with your O2 sensors, but there are a whole lot of other items to check...

 

I should have clarified, I have the BHR mid pipe with cat. That component is not new, but the CEL is. Coils, plugs, and wires are all new OEM a few hundred miles ago. As of June, compression was good, and my last tank of gas (after new ignition) was >22mpg. It's consistently been 21 prior. The car has been raced regularly but well cared for since new.

Before I start swapping parts, starting with the midpipe/cat, are there any other likely culprits?

 

Huh?

22mpg is considered good, and indicates that nothing that hurts gas mileage is currently failing.


Assuming that you are only referring to the P0420, and not the good gas mileage...

The fact that you have the CEL you do means that you have some or all of the cat element missing, letting through unfiltered exhaust. Aftermarket cats don't last long with the RX-8, and while BHR offers the catted version, they don't recommend it for that reason. One member blew out the cat in a BHR midpipe in about 90 seconds.

If you need a cat for emissions reasons, you have to go with a stock cat. If you need a cat for only smell reasons, then there is a magnaflow part number that will hold up enough to justify the cost/trouble of having it welded into whatever pipe.


There is no other reason for a P0420 other than unfiltered exhaust.

 

Thanks.

22mpg is what I got previously, now I'm looking at <60mi per quarter tank (I haven't filled up since the CEL popped up).

I need a cat for autocross (STX). This may be the excuse I needed to finally get tuned.

 

The RX-8's fuel gauge isn't linear, and it has a lot of electronic damping in it to prevent random needle swings, and the sender is in one side of the split fuel tank, not both.

Don't panic about gas mileage until you have most of a tank gone and it's virtually impossible to get back to where you were, or multiple tanks that are consistently lower.

A quarter tank of change isn't anything you are going to be able to diagnose with.

 

There are sender units in both tanks.....

 

Hmm...

I thought I could only find 1 in the documentation. I may be wrong though. Guess i'll go looking again.

 

LOL...I've had them apart...so seen them for myself

 

Yup, you are right. Well, instructions for 2 different senders anyway, presumably there is one in each side.

My mistake on that.

 

This was indeed due to a failing cat. The mileage drop (ended up at 19.5mpg) was the result of catalyst being loose and partially obstructing exhaust flow.