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Ron, ASE Certified Technician
Category: Mazda
Satisfied Customers: 35605
Experience:  35 years as a automotive tech
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Just brought an 04 Mazda 6 with 99K. When driving home CEL

Customer Question

Just brought an 04 Mazda 6 with 99K. When driving home CEL came on with code PO431. Reset the code and the car was fine for a week. After a week, we were driving and the car lost power immediately. RPM went up with loss of power. CEL started to flash. When we checked the codes we had PO300 (a missfire) PO304 ( misfire on cyl 4) and again the PO431. I had the car towed (200 dollars at 3AM) home. Car obviously has a miss at this point. I know that cyl4 could have a plug/coil issue but coupled with the cat/o2 code, is it possible that my precat is clogged? How can I dignose this before spending thousands on taking to to a dealer?
Submitted: 1 year ago.
Category: Mazda
Expert:  Ron replied 1 year ago.

Hello and thank you for your question,

The most common cause for the code P0431 to set is usually a left side Catalytic converter ( efficiency below threshold) . Before you consider replacing it you should monitor the front and rear oxygen sensor voltages and compare them while at idle and at a steady cruising speed. The catalytic converter is likely bad faulty if the downstream sensor is switching voltage from rich to lean (0.9 volts - 0.1 volt) in a manner like the upstream sensor.

The misfire codes is as you suspect and in most cases the problem is either a failing spark plug and or coil with the coil being pretty common. The best way to check the coil is to swap it with one of the other coils and see if the misfire moves. If it dose then replace the coil. If it dose not then replace the spark plug.

Customer: replied 1 year ago.
Ive been reading that the cat or precat goes bad in these cars a lot and that it can ruin the engine if it sucks back in debris through the EGR valve. The car drove perfect until on interstate last night when the CEL started flashing and the engine felt liike it was missing badly. How can I test if the cat truely is clogged? Will a standard compression tester work if I plug it into the O2 sensor hole?
Expert:  Ron replied 1 year ago.

The cat is not likely clogged yet but has lost its efficiency and if left alone it will likely restrict at some point. If you compare you HO2S reading and the rear matches the front then its a very safe bet the cat is bad. Its not likely coming apart yet. In most cases if it has come apart you will notice a major loss in power.

Customer: replied 1 year ago.
there is a major loss of power. We were going 70MPH and after this, we couldnt get past 45 to limp it to a rest stop 1 mi away. I had to have it towed because of the loss of power.
Customer: replied 1 year ago.
I was more hoping the loss of power and misfire were related to the coils ...but after research, how it happened (as in fine one minute then boom) leads me to believe that it may be the cat. If I plug a pressure tester into the O2 port at the very front of the engine near the radiator and it reads back pressure...this indicates a cat problem correct? If a cat problem is indicated, would it be the front pre cat (one I can see at the engine front) or a cat further under the car? Also, which cyl is cyl 4 in the 3.0V6? If the cat was bad...could this cause an issue in cyl4 which resulted in the misfire code?
Expert:  Ron replied 1 year ago.

It could also be the fact that the coil is failing that the converter has been damaged. I would get the engine misfire cleared up and then clear the code and see if the Cat code returns. If it dose after is no misfire then its a pretty safe bet the Cat is failing.

Expert:  Ron replied 1 year ago.

Im think when its all said and done you are going to end up replacing a coil and or spark plug as well as the converter. Its a very common failure on this car. Please take a moment to rate my service so I can get credit for my time, the question remains open for follow up even after doing so, all you need to do is return to this question if need be.

Thank you again.