Check Engine Codes p0300 and p0421

[seasidemx5]

I have seen several posts here and around the web about these check engine codes, and would like a little more information. First, some background on my car. It is a 2002 Protege 5 with 130,000 miles. I was driving approximately 40mph and my car misfired and sputtered. The check engine light came on, flashed a few times, then stayed on solid. It began to run fine after the initial sputter. The code at that point was p0300. My mechanic checked the car over, couldn't find anything too obvious, so suggested that I run some fuel system cleaner through the tank, which I did. Afterwards, reset the code to see what would happen. Of course, the light came back on with codes p0300 and p0421. I have seen these codes, and this combination as I have been searching online for the cause. I have seen people change all kinds of parts, but would like to know from those of you who have solved the problem, what fixed it? It seems as though coil packs and/or precat are mentioned a fair amount, but I would like to know if this actually fixed the problem for you. Thanks.

 

[hotcracker69]

when was the last time you change the spark plugs?

 

[MazdaSpeeder]

Plugs, coils, wires. They're not too expensive, in a car with that many miles they should be changed anyway. That's most likely your p0300 code. The p0421 is cat related I believe, and you may have damaged your cat with the misfires. You can install a non-fouler, which tricks the car into thinking all is well, or you can look at replacing the cat. But, I would do plugs, coils and wires first. Solve the misfires, then deal with the p0421.

 

[seasidemx5]

The plugs and wires were changed 20,000 miles ago. I haven't done the coils yet--just wanted to make sure that may fix the problem before I began randomly replacing parts. I'll look into that, and I have used a non-fouler on another car. I'll see how the coils go, then work with the emissions. Thanks.

 

[outofluckchuck]

Summary of all the postings about P0300 and P0421

I've looked through all of the posts and this is the order you should replace your parts. It's listed in order of most effective and cost-efficient fix. I'm on steps 5 and 6, cuz I ran with a blinking CEL for 70mis (only way home ). Costliest mistake of my life.

1. Tune-up (replace spark plugs)
2. replace spark plug wires
3. replace/inspect coil packs(x2)
4. clean maf sensor
   1. buy cleaner
   2. spray
   3. reset the ECU - remove negative terminal for about 10 minutes
5. replace (or clean) egr valve
6. replace pre-catalytic converter (may have burned out precat - DEALER ONLY for 2002 OEM and no aftermarket available for 2002 in CA [i'm on the same boat]) or prefouler if out of CA.
7. replace (or clean) fuel sock
8. replace fuel pump (180k seems like a reasonable life for one of these)
9. replace o2 sensor
10. replace 2nd cat

I got plugs, Spark Plug wires, Coil Packs, MAF Cleaner, and EGR from the local Autozone (most had to be ordered). Did steps 1-4 and ran without CEL for 150mis, then I got thP0421- precat is shot. It's been sitting for a couple weeks til I can save some money for the ~$1100 replacement. The mazda only precat is $950 w/ gaskets and IS THE ONLY SOLUTION IF YOU LIVE CA for precat replacement. Hopefully you live in TX or something and you can do the prefouler...

My advice for any P5 owner, replace your coilpacks and spark plug wires when you do your timing belt replacement or at 125k. It will spare your cat and save you $$$ in the end.

Good luck to you

 

[seasidemx5]

Thanks for the info--that definitely helps. I replaced the coil packs and the P0300 code has cleared. I still get the P0421 code, so I'm going to look at the exhaust options listed above. One down, one to go.....

 

[GlowstickNinja]

You can do what I did, install a non fouler and some OBX headers. Get more horsepower and solve the issue. Non fouler costs about 12 bucks max, OBX headers can be had online for 60 bucks or so. Hopefully you don't live in Cali, then you have to buy the pre cat, that's like 300 bucks.......

 

[tweety2300]

or do what i did do the non fouler and deal with the rotten egg smell. either way unless your rich or live in cali look into the non fouler

 

[fsbi2]

I'm having similar issues with these two codes. I have a 2002 P5 and I've already changed the plugs and wires (4months ago), cleaned the MAF, checked the EGR valve (dirty but still moving freely), cleaned the fuel filter sock, and replaced the O2 sensors. My next step is to replace the coils but I'd like to be sure they are bad before I do it. Other than physically inspecting them for cracks ect. how can I test them? Measure resistance maby? where?

Thanks

 

[GlowstickNinja]

I bet you have a bad pre-cat. How many miles on the car?

 

[fsbi2]

The car has 140,000 miles. Just checked the coils by pulling the connector off and got a consistent blue spark on both. Also checked the plugs and none of them seemed fouled. Would a bad pre-cat cause the misfire issue? Also, we have run the car quite a bit with the flashing CEL so I'm guessing my cat is shot.