Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

DEQ fail P0421 - Warm up Catalyst Efficiency Below Threshold (Bank 1)

Failed the DEQ test with a trouble code P0421. Had a local mechanic do a cat test, he said the cat was clogged but using premium fuel whould clear it up and to try DEQ again. Failed DEQ again with the same code and then got a cat test by a franchise mechanic, they said I had to replace the cat and oxygen sensors. The fanchise place had a pretty high service charge so I priced another local mechanic and he said that my vehical shows two cats, one in the intake manifold and one in the exhaughst pipe and he reccomends doing a diagnostic test to see if it is one of the cats or just the oxygen sensors. Now what?

There is no cat in the intake manifold. Either you misunderstood him or he missed his AA meeting.

Ther are vehicles that have two cats, on ein the exhaust manifold and a smaller one in the exhaust pipe, the second one not necessarily monitored. Could this have been what he meant?

Seems like I’d let the local mechanic run some tests. The frachise place is just replacing lots of parts figuring one of them will take care of the failed DEQ test.

More info on the vehicle; make, model, year, motor size, # of miles, etc. might help.

There is no cat in the intake manifold. Either you misunderstood him or he missed his AA meeting.
Ha ha. Yes I could have heard him wrong. It sounded like what he said but maybe not.
Seems like I'd let the local mechanic run some tests. The frachise place is just replacing lots of parts figuring one of them will take care of the failed DEQ test.
I was suspicious of that. Especially since the 1st local mechanic didn't say straight out that I needed to replace the part. The other local mechanic said he could run a diagnostic test for $100 and then know what is causing the problem. If it is just the oxygen sensors then that would cost me around $200 for parts as opposed to $800 to replace the manifold catylitic converter.
More info on the vehicle; make, model, year, motor size, # of miles, etc. might help.
2002 Mitsubishi Lancer ES 4 cyl. 230,000 miles

Maybe what he was saying was that there are two oxygen sensors, one before the cat (just after the exhaust manifold), and after the cat. That’s a common configuration. Before continuing, try to get some clarification from the guy on this point. Three might be two cats, but it wouldn’t make sense for a cat to be on the intake manifold. Cats are for cleaning the exhaust gasses, not for cleaning the intake air.

I’m not sure what the DEQ test is but I’m guessing it is an emission test. Deleware Emissions Qualification or something? Anyway, what’s the association with P0421? Do you mean your “check engine” light is on? In most areas if it is on, the car will automatically fail emissions. Try Googling “Mitsubishi Lancer P0421” and see if you find anything. Often if there is a common problem with a particular make/model, other folks in forums around the internet have already discussed it, and Google will find these discussions for you. They may have already found the solution.

There’s other engine problems that could cause this, but based on what you’ve said however, and given the 230K miles you’ve clocked, my first guess would be that you need new O2 sensors and a new cat. New properly calibrated O2 sensors will aid your engine computer to set the proper combustion mixture. A proper combustion mixture goes a long way to cleaning the exhaust gasses even before they get to the cat.

The cat(s) simply cleans up any minor (if the O2 sensors are doing their job) exhaust gas problem remaining after properly calibrated combustion. I think you probably need a new one. They’re supposed to work at least 100K I think, by Federal mandate, but 230K is stretching it for one cat. If you do decide to get a new cat, make certain it is a type that will satisfy your state’s requirements. There’s cats and then there’s cats in other words. The cheaper ones don’t work as well and don’t last as long generally. Some states – like Calif in particular – are very obsessive what kind of cat is permissible, and if you use the wrong kind, you’ll fail the emissions test irrespective of what is coming out of your car’s tailpipe. So double-check that before ok’ing any new cat install work. Best of luck.

Thanks for the comments and suggestions. DEQ is the State of Oregon Department of Environmental Quality - vehical emissions inspection program, this is the first year I have lived in an area that required inspection. I have to pass the inspection to be allowed to get my car registered with new plate stickers. I have a California Emissions type cat in my vehical so that makes replacement most expenisice.

Mechanic #3 that I spoke with today was the one (the only one) that said that my car had 2 catalitic converters. What he had told me was that a diagnostic would be best as the “manifold catylist” would cost $750 in parts + labor, the catalyist “in the exhaust pipe” would cost around $380 + labor".
All mechanics reccommended that I replace the oxygen sensors. Maybe I’ll start with oxygen sensors and see if it passes the emissions test. The car does have quite a few miles and has been driven all over.

Ya know I think I had that code on an 03 windstar and it ended up being plenum orings, Became a new persona and cannot find the old posts, try a dealer is my suggestion as the explanation was they can read more codes.

I am assuming that the P0241 should read P0421. There is no P0241 for your car and the P0421 fits the situation you are describing.

The P0421 code is set if the quotent of the frequency of the rear O2 sensor divided by the frequency of the front O2 sensor is above 0.8. This indicates that the front sensor is not active or the two sensors are tracking in parallel. The two sensors monitor the upstream catalytic converter so this code does not relate to the down stream catalytic converter. The failure of the down stream catalytic converter can only be checked by doing an tail pipe exhaust measurement.

A Mitsubishi service department should have a scanner that can monitor the activity of the rear and front O2 sensors. If the rear O2 sensor were not active, IIRC another P code would be set. If the front O2 sensor is not active, its replacement might solve your problem. If the scanner indicates that the front and rear sensor are tracking together, it would appear that the exhaust manifold up stream catalytic converter has lost active surface and must be replaced to pass DEQ.

Hope this helps.

@AveryBay …also be sure to Google “How to pass an emissions test” or “How to pass a California (or Oregon) emissions test”.

My car tends to come close to not passing, so when my inspection is due, I do everything I can think of beforehand to increase the odds it will pass. This has been working for 20 odd years, but the last few years, just barely. I found those sites to be useful that I found with the Google search. As I recall there’s a couple of websites. They offer practical things you can do to increase your odds, such as putting in a new air filter, making sure the car is fully warmed up, having a full tank of gas, driving the car at freeway speeds for an hour or so to clean out carbon buildup, etc.

The two catalyst system is common to some cars. But this usually makes them last longer. The O2 sensors are the only data that anyone uses today unless the drag out the emissions sensor dino from the closet. Try the O2 sensors to start. Also check for bad fuel injectors. I normally replace all FI at 100k but you did not say your miles. Fuel injectors can be as much as 60$ a piece but for mileage they make your economy and save the catalyst.

The front O2 sensor may be getting lazy and is triggering the code. Before replacing the CATs it would be a good thing to start with the O2 sensor. If that doesn’t help then before you replace the expensive CAT you might try using a product from BG Products Inc. ( It is called BG 44K Power Enhancer. It will help clean out deposits in the fuel system and help burn out any deposits in the CATs which may be causing the issue. You add the product into fuel tank before you fill up. The part number is 208A for the bottle and 208 for the can. It may take a couple of treatments to do the job.

Excellent suggestions. I will try the O2 sensors first and see what happens. With 230,000 anything is possible. I have never had trouble with is though. It’s 10 years old and running so well.

Step One: Replace the REAR oxygen sensor, ($80-$100) reset your cars computer to turn off the dash light and see what happens. If the light stays off for 2 days, go get your emissions test. If the light comes back on…

Go to an independent exhaust system shop, have a new generic CAT installed ($300?) and go get your emissions test…You should be able to get past this for $500 or less…

I guess I would suggest the dealer to just to diagnose. My car has two cats. One for the front bank or front three cyl, and one for the rear bank or rear three cyl. Don’t know which is bank 1 or bank 2. The pipes then join together in one exhaust pipe after the cats and into the muffler. There would be two O2 sensors for each cat. One before and one after. Each of my cats is about $700 for the part. Don’t know what yours has though, just have to give it a careful look from above and below to see.

You said in a previous post it was a 4-cylinder car…If so, there will only be 2 oxygen sensors, front and rear.Four cylinder engines only have one “bank” of cylinders…Don’t worry about the pre-converter in the exhaust manifold, just the one in the exhaust pipe…

"2002 Mitsubishi Lancer ES 4 cyl. 230,000 miles "

The preconverter is used to heat the cat faster. It is designed to burn raw fuel to heat the exhaust to cat temp faster. If this one is slow to heat the system it may cause this code. The computer may push this code due to a slow heatup of the cat. The o2 sensors are a cheap start though. You could also have leaking fuel injectors that are ok when the engine is hot but throw a code when the engine and the converter is cold.

I found out a little more. First the check engine light bulb is supposedly broken so that is why I didn’t see the light. Next the DEQ sheet showed the following:
Misfire, Fuel system, component, catalyst, Evap, O2 sensor, O2 heater, EGR/VVT = READY
Catalyst Heater, 2nd Air, Reserved = Unsupported
DLC, Comm = Pass
MIL = Fail