I have a 1998 Mazda Protege LX 1.5L. The check engine light has been on forever (seriously ~80k mi, has 100k on it now). It has passed emissions before (1999, 2001, 2003, 2005), but now they have changed the law and they can't test it with the CEL on. I'm trying to sell the car and may sell to someone I know, so I don't want to just reset the code and sell a lemon, I want to do the right thing.

So, I read the code yesterday and came back with one code, P0421. I've tried researching what this could be on this site as well as others and the most probable explanation seems to be an O2 sensor. Based on information, I provided, does that make sense? I have read that other Mazda's may have the same problem and a flash of the PCM fixed the problem and I was wondering if that could be a possibility with my model/year.

Assuming that the problem is the O2 sensor:

I believe there are 4 sensors (2 pre-cat, 2 post-cat). Which sensor does this particular code refer to? Also, I don't know that much about cars so if this could be in layman's terms it would be helpful b/c I think I get confused b/t front and rear.

I have read that some Mazda's have thrown codes due to an oversensitive O2 sensor. If I replaced with the OEM model, is this likely to happen again, because it is the same part?

I would spend extra money to get an O2 sensor that is ready to go, b/c I don't want to solder. Any links, part #'s?

Thanks.

 

1998 Protege with the 1.5 Z5 engine would not be that uncommon to have a weak/failing front warm-up catalytic converter that would set that code. Most dealerships probably even have one in stock as they are so common.P0421 is specific to California/Green state emission level vehicles. P0420 is used for Federal emissions. Very rarely is an oxygen sensor the cause of a Cat efficiency code. Both oxygen sensors ( Your car does only have two ) have their own codes for various faults. The warm-up Cat is bolted directly to the outlet of the exhaust manifold. The front O2 sensor is mounted directly into the exhaust manifold and rear O2 sensor is mounted into the short little 'L' shaped pipe that is right under the engine. This rear one is the one used to monitor the efficiency of the Catalytic converter. I know you have passed your emissions tests for many years but keep in mind that you also have a very large Catalytaic converter further down in the exhaust system that does most of the work to clean up tailpipe emissions. The front one is there to start dropping the emissions level after cold start up as it can heat up much quicker being that close to the manifold and hotter exhaust gasses and is the only one currently required by Government standards to be monitored for efficiency.
Make sure that you do not have a crack in the exhaust manifold or that little 'L' shaped pipe as this can also dillute the exhaust and cause some similar problems. If no exhaust leaks then your best bet would be to install a new warm-up converter. They are not very expensive at between $265-$284 plus gaskets from Mazda. Could even get for less from www.onlinemazdaparts.com but you will need to know if it is Federal or California emissions as they are different part numbers.
Too bad you did not get checked back when it came on as emission warranty could have covered the replacement for a period of 8 years/80,000 miles.[:@]

 

DOH!!! You're right, I should have had it fixed under warranty. My wife actually took it in with very few miles on it and they said it was a common problem, it didn't matter, they reset the code and said it would probably come on again. Should have pushed them to fix it.

Anyway, this is probably a dumb question, but how do I know if I have a California emission level vehicle? I know I don't live in California, I live in Northern Virginia near DC. It was definitely a P0421 code, so does that mean I have a California emission level vehicle and would have to buy the appropriate parts for CA?

Also, I went on www.onlinemazdaparts.com. I didn't see anything that said "Warm-up Converter". I saw catalytic converters under Replacement Parts/Exhaust System/Catalytic Converter but I'm assuming those are the larger ones farther down the exhaust system you had mentioned. I also saw a manifold gasket, was that the gasket you were referring to. I think you had mentioned these are bolt-on parts (not weld on), is this correct?

I'll check to make sure there is no leak in the system, then hopefully can get the right parts ordered and give it a shot.

Thanks so much for your help. I really want to learn more about cars and repairs and it is extremely helpful to get advice from someone very knowledgeable instead of blindly guessing and making costly unnecessary repairs.

 

Now that is ironic. Came into work this morning and there is a 98 Protege with CEL on and P0421 stored. Thought it might be yours untill I saw you were in VA. It is a Federal so looks the shop manual is wrong on that one.

You can tell if it is a Federal or California by the sticker on the underside of the hood. Top left of the sticker should say " Vehicle Emission Control Information". Straight down from that at the bottom but above where it says in large letters "CATALYST IDENTIFICATION" There will be a box with writing in it. Federal emission cars will read " This vehicle conforms to US EPA regulations applicable to 1998 model-year new motor vehicles. California emission level will iinstead say "conforms to California regulations.

If you do have Federal car then the use the "Search by part number" box instead. Warm up CAT is a Z502-20-500. Inlet gasket is B6CK-20-520. Outlet gasket is KL19-40450. Gaskets are the same for California car but the CAT is a B6GD-20-500C.

Took a few pics of a Catalyst Monitor Capture to give an idea of what it looks like. These show the voltage changes of the front O2 sensor on the top and rear O2 sensor on the bottom. Scale is set at 0 to 1 volt on both sensors and duration is for 30 second time period. The blue line is realtime voltage from both O2 sensors. When line or voltage is at the top it means a rich condition or low oxygen content exhaust. When at bottom it means a lean or high oxygen content exhaust
The first one shows a normal, rapid fluctuation in voltage from high to low with both sensors when engine is held at a steady 2500 rpm. Second pic shows just after dropping down to idle and a good catalytic converter. Note that the bottom or rear O2 sensor is steady and on the high/rich/low oxygen content side. This means that there is very little oxygen left in the exhaust after the Converter so the GOOD converter is using up most of the oxygen left after combustion to clean up the exhaust.
Last pic is BAD converter because the rear O2 sensor is fluctuating at the same rate as the front O2 sensor. BAD converter is not using oxygen to clean up the exhaust and needs to be replaced. This one is actually very bad because rear O2 cycles from rich to lean at a 1:1 ratio with the front O2. The PCM will actually condemn as not being efficient with as little as a 2:1 ratio.

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Thanks for the part numbers. That helps tremendously. I forgot to check last night, but I'm assuming I have a Federal car (I'll double check though).

Funny you had a car with the exact same problem. Would you say this is a common problem?

Any tips/advice you can provide? Any special tools required? I would consider myself a novice, and only have basic tools, sockets, etc. Only thing I've done like this before is put on a Borla Cat-back exhaust on a '96 Camaro Z28.

I'll probably order the parts in the next few days and probably put it on next weekend. I'll let you know how it goes.

Thanks again for all your help.

 

There is atrick I have used to pass the emission tests. Before southernOhio stoppedemissions testing, I would just drive to within a mile or so of the testing facility pull over in a parking lot, turn the car off, then unhook the battery.Taking the power away from the systemshould wipe out the code in the computer. Then hook the battery back upand go get tested. Unless it is a serious engine problem it will take a few starts before the codes will pop back up. It always worked for me.