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Doug C.
Doug C., ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 6171
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi Lancer ES how to fix p0421 code

Customer Question

how to fix p0421 code?
Submitted: 2 years ago.
Category: Mitsubishi
Expert:  SuperDave replied 2 years ago.

SuperDave :

the most likely cause to your problem is

Catalytic converter deteriorated

Heated oxygen sensor failed

ECM failed

PCM failed

SuperDave :

your code is a downstream o2 sensor code

Expert:  SuperDave replied 2 years ago.
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Customer: replied 2 years ago.
i take the car to 2 different mechanic,1 told to replace cat converter,the other to replace 02 sensor downsteam.how do i know,which one i have to replace
Expert:  Doug C. replied 2 years ago.
Hi,

To know which one to replace, you need to understand how the system is monitored.

The catalyst monitor the PCM uses consists of the exhaust manifold converter (NOT the converter under the car), the front oxygen sensor and the rear oxygen sensor.

As the exhaust exits the engine, a reading is taken at the front oxygen sensor to determine oxygen/fuel content of the exhaust. The oxygen content generates a low voltage reading that is reported back to the PCM. This voltage will fluctuate as oxygen levels fluctuate, and due to the PCM constantly adjusting fuel control, will result in a fairly rhythmic voltage fluctuations between roughly 0V-0.2V and 0.6V-1V.

The exhaust now enters the catalytic converter portion of the exhaust manifold. Here unburnt hydrocarbons are converted, altering the air/fuel content of the exhaust.

The exhaust then exits the exhaust manifold converter and is read again by the rear oxygen sensor. The PCM now compares the two sensor readings to determine if the catalyst is working. If the sensor readings are extremely similar, this is a warning sign that the catalyst is failing. If the two sensor readings are fluctuating identically, the converter is failed.

What you would want to look for here on a scan tool is that the front sensor is fluctuating properly at idle between 0.2V and 0.8V roughly, and every 1 second or so. The rear oxygen sensor you will want to see between 0.4 and 0.8V roughly, and a -fixed- voltage... meaning very little fluctuation/deviation.

If you see a front oxygen sensor that is stuck at one voltage, the front sensor is at fault.
If you see a rear oxygen sensor that is stuck at very low voltage (0V, 0.1V etc) the rear oxygen sensor is at fault.
If you see the front sensor fluctuating normally and in range, and the rear oxygen sensor fluctuating as well, and within 80% of the front sensor voltage for more than 12 seconds or so, the catalyst is failed.


It is important to note that this vehicle had a software recall issued on most models to change the fueling to prevent premature catalytic converter failure. Initially they were replacing the converters for free as well, however at this age the converter replacement offer has expired, but the software update is unlimited time/mileage if your vehicle fits into the production range (most do) and has not had the update performed yet.

It is also important to note that the catalytic converter that is monitored is the one built into the exhaust manifold NOT the one below the car. Aftermarket auto-parts stores will try very hard to sell you the catalyst under the car because it is much cheaper and probably the only one they can find. The catalytic converter under the car is NOT monitored and it is impossible for it to trigger a code.
Customer: replied 2 years ago.
I don't have tool and skill to scan the car.
If I'm replace the o2 sensor(downstream) and replace converter in manifold it will fix the problem, I'm try to DIY

And i know this has been recall and I'm already take to dealer
To get it fix q back in 2005
Expert:  Doug C. replied 2 years ago.
Okay if the software update is done that is good. As far as determining the cause without a scan tool, you are a bit out of luck there.

As I mentioned before, all three components can theoretically cause this code to come up.... if the front O2 sensor sticks at a solid voltage, it will appear as if the converter is failed when it hasn't (since the exiting exhaust is going to create a fixed voltage as well, it will appear to similar to the monitor).
If the rear O2 sensor is stuck at a very low voltage, it will be out of tolerance range and throw the code.
And of course if the converter is chemically degraded, that will cause the code to come up as well.

For the sake costs, I would strongly recommend taking it to have the sensor readings checked so you know for certain what you are dealing with. It should not exceed $45 to have this checked by a professional (Dealership, respected independent shop etc... not Sears, Tire Stores, etc).

The majority of the time... and by majority I am talking over 90% of the time, the only problem you will have is the exhaust manifold converter failing if P0421 is the only code you have. It is very rare to see either O2 sensor cause this code --on this particular vehicle--. On other vehicles, yes... but on this one it is almost always the converter. Knowing that, changing the converter and rear sensor as you describe would almost certainly have you covered, however it would be terribly unfortunate if you did all that only to find out the front sensor is stuck and causing the problem all along and all that expense was wasted.
Granted, this is not terribly likely, as like I said it is almost -always- the manifold converter and the sensors test out fine, it is just safer practice to have it verified 100% rather than speculate even if the odds are very strong in one direction. If you must replace without diagnosing it on a scan tool, I would not bother with the rear sensor yet, start with just the converter as odds are that will be it and you will save quite a few dollars if the rear sensor was fine.
Customer: replied 2 years ago.
Do you have part number for converter?
Just want to make sure order the right part

And how long o2 sensor will last before fail
Expert:  Doug C. replied 2 years ago.
Sure... there are two types of converter you can order, the regular one, or the "Recall" one which is at a reduced price (for internal purposes). I always get the recall one as it is cheaper and comes with all new gaskets etc as well.

For a 2002 you will want to order part number MN195801. This will include all gaskets and new nuts/bolts.
List price is $402.23, and would be what you would pay (at most) if you went to your dealership to order it. You can order online (www.mentormitsubishi.com for example) for significant discounts as well.... Mentor normally charges about 25% less than list price. You will need to order by part number if you order online... do NOT use the parts catalog as it will give you a much higher price.

The O2 sensors are only common for one type of failure on this vehicle and that is heater circuits blowing out... where the sensor continues to work normally, but simply does not pre-heat on key-on. They are hit or miss, I have some customers that are nearing 200k miles on the original sensors, and others that fail very early (I have one customer with a 2006 Lancer with 13k miles that had a sensor fail a few weeks ago). It is entirely hit or miss, and because it is normally a heater failure, it can not likely be attributed to anything other than manufacturing defect.

Over all I would expect 60-80k miles out of the O2 sensors, that is a fairly safe expectation... some last longer, some fail sooner, but 60-80k is about average.
Doug C., ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 6171
Experience: Mitsubishi employed and Factory trained ASE certified technician
Doug C. and 4 other Mitsubishi Specialists are ready to help you
Customer: replied 2 years ago.
What the different beetwen recall one and the regular. , because they can't even tell me when they will have in stock.
Expert:  Doug C. replied 2 years ago.
Hi,

There are two differences between the recall converter and the "regular" converter that you would get if you ordered from the parts catalog:

1) the recall converter will come with a new gasket for the head and tail pipe, new bolts for the tail pipe and new nuts for the head studs. The regular converter is just a converter and you would have to order the other parts separately.

2) the cost of the recall converter is much lower than the regular converter, typically about 1/2 the price (in your instance about $400 versus $920). This is because this is the part Mitsubishi uses for vehicles under warranty, and the lower the price is on it, the better their botXXX XXne looks on warranty repair costs.

Essentially, the recall kit was made to be a single part to order that came with everything, and have a lower cost so that Mitsubishi can "bill themselves" less money for the repair.


The recall kit that I provided the part number for above is WELL stocked and should only be a few days for any dealer to order. Checking out warehouse inventory, the Atlanta warehouse has 127 of the kit, New Jersey has 58 and Los Angeles has 130. In addition to that, there are about 100 dealers that have the converter in stock on their shelves as well. Ordering this part will NOT be a problem, I have no idea why they would indicate that it would be. If they give you a hard time, tell them you had the warehouse inventory checked on parts-on-demand and there is no shortage.

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