So I think my catalytic converter is going bad P0420

[SM16RMSM]

Well, ever since I bought my MSM, I don't really get good MPG, top is 22 MPG and that is in City. Highway is even worse. Well, the other day my CEL came on and I took it to a friend shop. I hooked up his computer and it gave me a P0420 code. Well, it says "catalytic converter efficiency below threshold bank 1". I cleared the code for now and we watched the volt reading on the scanner tool. He is saying my readings are right at the threshold point to make the CEL come on. He think that I have some bad gas or a bad catalytic converter. What do you all think?

Also, my MSM only has 38,5xx miles too.

[—RotorNutFD3S—]

Stock cat or aftermarket?

Regardless, the OEM NB ECUs are notorious for throwing this code. Most people get tired of fighting it and either install o2 simulators, spark plug anti-foulers, or some other trick to satisfy the ECU. But your lower fuel economy might mean that you have some other issue going on.

[— Agent☣Orange —]

Could just need an intake/EGR cleaning. If not that then the front O2 sensor is going out. I don't think he's changed anything since he got his MSM last year but it's a guess.

[SM16RMSM]

Yata right, It still 100% stock except for my HID and K&N filter in the stock box. That is about it. As for the EGR, it was cleaned around 30,xxx. I did Iridium KGN plugs and Megnecor wires during that time too when I cleaned the EGR out. As for the fuel economy, it been like that since the day I bought it. I suspect the diverting valve is leaking to much. From what I am told over on M-S, the stock valve leaks so badly that within one year of ownership, you basically lose 1 PSI, and it continues until it doesn't hold boost anymore.

[SM16RMSM]

I guessing no one esle wants to chime in on this subject

[—atank—]

http://www.caranddriver.com/reviews/...iata-road-test

FUEL ECONOMY:
EPA fuel economy, city driving: 20/26 mpg
C/D-observed fuel economy: 21 mpg

[—atank—]

Looks like you are right on with the fuel Mileage, we are both in the same boat!! Are you running those Factory Racing Hart Boat Ancors? 2004 Racing Harts are 17.1 lbs and the 2005s are a piggy 18.4!



http://www.fueleconomy.gov/feg/bymod...-5_Miata.shtml

http://www.mazdausa.com/MusaWeb/pdf/..._specs_MSM.pdf

[brandini]

I get decent mileage in my msm and it's been pretty much trouble-free:

[—atank—]

So thats a total average both city and highway, looks good!! My driving is 80% uphill - downhill in these North Georgia mountains!

I get decent mileage in my msm and it's been pretty much trouble-free: