2002 Mazda Protege 5 - P0421 & P0300 OBDII CODES

How bad is it to replace the fuel pump? I am starting to get random PO300 codes as well, so last night I replaced the coil packs and cleaned the TB, idle air control valve, and EGR. I have yet to replace the intake tube, but now I'm thinking I should get me a fuel pump as well. It's definitely driving much smoother now, but after 1/2 an hour of driving I took off from a light and the car stuttered for a sec and then got going. I'll probably replace the fuel pressure regulator as well while I'm at it. Has anyone straight-piped the pre-cat and then pulled the first O2 sensor out of the exhaust stream a little to fool the computer? I did this on a turbo legacy and didn't throw any codes and passed DEQ this way.
 
flashing cel came back today, two weeks after fuel pump replacement. Haven't gotten codes yet, but I am betting its p0300 again. Fuel pump replacement is fairly easy, parts cost about $150.
 
Problem was multiples but mainly cause by a hole underneath the cold air intake hose. Couldn't see the hole. Would flex and let more or less air in causing fluctuations in air intake. Also coils were bad. In turn causing precat to go bad. Replaced all and she runs great.
 
In March we were seeing odd idle RPM's and the car was sputtering. We took it to the dealer, who we won't use anymore, who adjusted the idle speed and ultimately put a new OEM alternator on the car...ouch. Tired of overspending money at the dealer while still experiencing the problem we were referred to a reputable mechanic in town. Just before we took the car in the engine would not start, it was turning over but wouldn't fire. Then we got at PO300 and a PO421 code, multiple times (this happened a month ago). After multiple trips and trouble shooting we found out the bank 1 cat was completely shot. So they replaced it with an aftermarket part. After that the car continued to not start right. So the mechanic gave us his extra car and kept ours for three days to trouble shoot it. I found this forum and emailed it to him. So they changed out the coils. Problem solved! The best part is because of it, the service manager and mechanics went to a class on engine coils for new vehicles, in which the master mechanic said new coils really can't be tested, so if they see customers with these problems it is most likely the coils.

So thanks to Made In China for posting the response about the coils!
 
My 2003 P5 has p0421 and p0300 codes. I know what they mean but am unable to fix the problem. I have replaced EGR valve, 2 O2 sensors, cat 3x, both egnition coils, spark plugs with complete tune up and timing belt change. spent a lot of $$$. Anyone have same problem and able to fix? I have 111k on my car. At first engine light goes on solid and I have less power.Sometimes it lags for a sec and returns power. right before cat goes, It loses power for minutes anywhere, anytime and then it is hard to regains any power. some times I will get down to 20mph and unable to gain speed. The light flashes at this time.



im having the same issue! i just got the code p01250 i believe, something about my fuel pressure and the IAC valve. the light went off and the next day I get this p0300 code. have you fixed this yet? i was thinking it is my catalytic converter. there is no power like it should and it sound a bit muffled like a ricer. i already checked my coils.
 
im having the same issue! i just got the code p01250 i believe, something about my fuel pressure and the IAC valve. the light went off and the next day I get this p0300 code. have you fixed this yet? i was thinking it is my catalytic converter. there is no power like it should and it sound a bit muffled like a ricer. i already checked my coils.

Code P1250 is a problem with your fuel pressure regulator or its circuit.
Low fuel pressure can cause a misfire.
Fixing the regulator or its wiring should fix your problem.

PS ,... If you read this entire thread you'll know that you can't check or test coils,... You just replace them,...
 
Hello All,
Mazda Protege5, 2002, 2.0L, 280,000Km.
Last week I got my first P0421 fault code and also briefly P0300. Before reading this thread, that it could lead to the destruction of my CAT, I cleared the fault code and opened my OBD2 app and recorded a video of both O2 sensors, the second column from the left. Sensor 1 was all over the place as you can see in the video, compared to sensor 2. I tried to keep the throttle at constant position during the video capture. There is no O2 sensor fault code or any other fault code.


Is this behavior of the O2 sensor is typical? The coils have never been replaced since new, so should I replace both coils first and ask questions later?
Thank you
 
Last edited:
I have the same p0300. Usually after slowing the car down and showing in a serious misfire / loss of power. Pull over the car. Lift the hood and pull one of the plug cables a bit so it archs against the block. The misfire clears and it runs like a top for a while. Rinse and repeat. Why does that arching clear the issue?

Sent from my SM-G900P using Tapatalk
 
I have the same p0300. Usually after slowing the car down and showing in a serious misfire / loss of power. Pull over the car. Lift the hood and pull one of the plug cables a bit so it archs against the block. The misfire clears and it runs like a top for a while. Rinse and repeat. Why does that arching clear the issue?

Sent from my SM-G900P using Tapatalk
Most likely coil packs are bad, very common issue in these cars
 
Just did new coils today. Almost no power for a few moments when leaving a stop, then it picks back up and seems to run Okay.

Does that indicate a possible vacuum leak?

Sent from my SM-G900P using Tapatalk
 
Same type of issue. I burned both coil packs earlier in the year and replaced them so I know they are good but to be sure I bought another and tried it. Packs are good. Arc and spark off pack...arc and spark off wire...arc and spark off wire and plug together on all. Engine runs rough like pack is dead. Started car and unhooked pack 1 (cylinders 2and3) car stays running but rough. Unhook pack 2 which is cylinders 4-1 and car dies. So I check cylinder three to find plug is black and not wet yet still sparking. Crank and run car with cylinder 3 open and no mist or fluid come out. Conclude cylinder 3 injector is bad. Looking for an injector now.
 
Back