Mazda 323 P0421
 

Hi All,

I picked up a '03 Reg 2.0L Mazda 323 last week. it had no issues at the time but around 100 miles later it popped up an error code (believe P0038). I cleared it and asked the old owner who said he had the same thing, cleared and it had not returned since. it did not come back for another 50 miles when I cleared again.

I drove around 100 miles after this and the engine warning light started flashing and I lost power fast. I pulled in, waited, and then was able to continue back. At this point I had 3 errors - P0443, P0300, and P0038. A mechanic had a look and pointed out a raised coil pack with a small crack. I got this changed to a new one and at the same time changed the spark plugs and cam belt.

Around 150 miles later the engine warning light came on. The car still drives smooth and this time the code is P0421. A google indicates it could be the cat or a sensor, could someone confirm how I might be able to check which one?

It's still driving very smoothly with no high idle or roughness at all. i hope it's the O2 sensor as obviously this is a much cheaper part and I've already put a fair bit into the car.

All help very appreciated!

Bump anyone? Just need to know whether this is going to cause any real issues and if so whether the likelyhood is bad sensor?

At the moment, I'm not at work & don't have 323 manuals online.
Looking at both 2.0, 2.3 & 2.5 Mazda 3s, DTC P0421:00 Warm up catalyst system efficiency below threshold

POSSIBLE CAUSE
• Leakage exhaust system
• A/F sensor looseness
• HO2S looseness
• WU-TWC deterioration or malfunction
• PCM malfunction


It's pointing towards a bad convertor, most likely the previous issues have blocked or deteriorated the cat.

Hi grim_reaper thanks so much for getting back to me.

Is there any way to check what's wrong before I get the work done?

If it is the cat what kind of cost am I looking at?

I agree with grim. Cats are just coated with the material necessary to treat harmful exhaust gasses. A misfire, (P0300, P0301-04), allows raw fuel to run thru the cat destroying the material. The older the cat, the less likely it is to withstand a barrage of raw fuel. If you have access to a scanner that will run live data, you can watch the reaction of the pre cat and post cat O2's, you will be able to determine the condition of the cat.

Thanks Sparky, that does make sense though obviously want to confirm first just to be safe.

I've just ordered a reader so I'll post figures on here when it arrives.

When watching the signals from the reader, depending on how it works, the pre cat O2 sensor will show high and low readings that should change at a steady rate like a wave. The post O2 should be at a steady reading. If the O2's mirror each other, that indicates the post O2 is seeing the same exhaust as the pre O2 and the cat is doing nothing. Dead cat.

Poor pussy.

Right so my OBD2 arrived today. It was a little bluetooth one so used the Torque app to check the sensors. There was several options - 02 volts bank 1 sensor 1,2,3,4 and o2 volts bank 2 sensor 1,2,3,4. I added all but only got a reading from bank 1 sensor 1 which showed the flowing diagram as mentioned before (between 0.7 and just above 0). If I'm getting no reading from bank 2 does that mean the sensors gone or that the car doesn't support this OBD2? Really hope it's the sensor!

You only have bank one in this engine. In-line 4 engines have only one bank: bank 1. V6 and V8 engines have two banks.

You should have Bank 1 sensor 1 - A/F sensor ( upstream / before cat ) & Bank 1 sensor 2 which is the actual O2 sensor, downstream/ post cat.

Thanks guys. I didn't have upstream/downstream on the menu but did have the aforementioned sensor 1-4. Assuming sensor 2 is the one we're talking about - that one did change to a reading when car started unlike others that stayed on blank (-) but it stayed at 0.00. Dead sensor or bad reader?

Scrap that. Tried again today after a drive. Results uploaded here:
http://postimg.org/gallery/d3t7znk2/9792e8fc/

So further testing... When driving along the graph is fairly similar between the two sensors. When still the second stays fairly steady and the other still goes up and down. Assume thats the correct way to text and it's not the pussy lives on?

Set up so your looking at bank 1 sensor 1, that's is what is going into the cat, and bank one sensor 2, that's what is coming out. Properly processed exhaust by the cat should read steady at 0.8v ish. Also, hold your RPM around 1200 to 1500. Takes a few minutes for the cat to fire off and everything to wake up.

Thanks sparky. Ok so took it for a ten min drive then tried it holding the revs between 1 and 1.3. Sensor two went between 0.5 and 0.7 and sensor one went up and down consistently. When not reving the engine sensor two stayed at a flat rate and sensor 1 continued to go up and down. What do you think?

If 2 didn't follow 1 like a mirror image, you may be looking at a lazy O2 causing the code after all. Long live the puss.

That's great news! Any guesses which sensor could be bad? If I just keep clearing fault code will I be doing any harm to car by continuing driving or is important to get sorted soon?

Found sensors on eBay quite cheaply so might look to get one there. Is it quite a hard job? I don't think I have the tools or know how to do myself but always good to know before I ring the garage how long they should be charging for, been fobbed off far too many times before!

The post, down stream, bank1 sensor 2, whatever you want to call it, appears to be the lazy one. O2 socket can be purchased without dipping into your Christmas account and changing it out is fairly simple.

Side note. Have had some bad times with "cheap" sensors.

Thanks so much for everyone's help. I'll get the o2 changed and hopefully that'll be the end of it.

Cheers all

Let me know the outcome. If I ever claim to know it all, feel free to thump me over the head with a rubber mallet. Still learning something new every week after all these years.