2000 Miata P0420 questions

02-24-2010, 08:11 PM
I have a 2000 Miata that I purchesed from an individual in October. Shortly after getting it home (2 hour drive) the check engine light came on. I went to Autozone and had the code checked and it came up P0420. After doing some research I am trying to diagnose whether it is the cat or the O2 sensor. Since Autozone only reads the codes I purchased an EZ SCAN 4000 AUTOXRAY OBD II SCAN TOOL (great tool by the way) to do some more thorough testing.

A little history on the car. Has 125000 miles, has been serviced regulary and all fluids are synthetic. The previous owner has installed an AEM intake with K&N filter, Magnecor Performance ign. coil and wires, and a Catback dual exhaust system. It runs fine and there are no vacuum leaks that I can find. I have read that if oil gets on the cat that it can damage it and am wondering if the K&N filter oil may have attributed to its demise. I also noticed that theses cars have had problems with the coils, which can also lead to cat damage. I am thinking that may be why the coils were replaced in the first place. All this was done before I purchased it so these are just assumptions.

In closed loop O2 sensor # 1 fluctuates between .110v -.725v and O2 sensor #2 is at 0volts. After vehicle is warmed up and running in closed loop Sens #1 fluctuates between .90v - .810v and sens #2 fluctuates between .80v - .800v (give or take). The # 2 sensor is fluctuating at right about the same voltage as as #1 and at the same rate. #2 follows #1 milliseconds after #1 and within 10-20% of #1 readings. I believe that this is indicating the cat is bad and not cleaning the exhaust because #2 is fluctuating so much, so quick, so often. I believe the voltage readings should be lower(because the ex gases are cleaner) and the fluctuations should be less often. If the cat was good the readings would be different between #1 & #2 and #2 would not fluctuate near as much.

Any input in this matter is appreciated as I don't want to start throwing money at it replacing unneeded items.

Thanks in advance

03-17-2010, 12:31 AM
that is most definitly a bad cat. sorry.

the post cat o2 should remain pretty much stable around .330 or so

10-12-2010, 04:14 PM
My friend has a 2000 Miata that was making loud screeching noises when he first started the engine with the air running. Now it just runs hot if you run the air. Lately I checked and the check engine lights up immediately and it starts to run hot. I'm wondering if maybe he overdid it the last time he tried to drive it. I bought the gasket and thermostat, but might be too late to try the small things. What do you think?

10-12-2010, 04:24 PM
The down stream HO2 sensor told the ECU, that your cat is gone!
see, http://www.catalyticconverterwarehouse.com
Give your car a good tune up, befor replacing the catalytic converter.

Spark plugs, Air filter, Fuel filter, clean throttle body, clean MAF sensor, and a New upstream H02 sensor.

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