Skip the auto shop - Our mechanics make house calls

Q: the car wont pass emmissions the code po420 comes up

asked by on

2004 Nissan Sentra L4-2.5L my car will NOT pass emissions due to the check engine light - code PO420 keeps coming up :( For several years the light has been on but my spec-v has always been maintenanced as recommended and there have been no major problems besides replacing the master cylinder? (For the brakes?) Last year I ran into the same problem but was able to get the car to pass but putting some Sea Foam in it to help clean in the gas tank, now this year I have had the codes cleared three times so the check engine light has gone off, for 32 miles the 1st time, 20 miles the 2nd time, and 8 miles the last time. I am not financially well to do, thus can not afford a real mechanic to tell me how to get this car to pass!!! PLEASE HELP!!!

My car has 173250 miles.
My car has a manual transmission.

A: Code P0420 is for the efficiency of the cat...

Code P0420 is for the efficiency of the catalytic converter (CC). You have two CC’s on your car. There is one built into the exhaust manifold, and there is one in the mid-pipe, under the car.

The one in the manifold is the one monitored by the computer for efficiency. The O2 sensors are used to monitor it. The CC is an important emissions device. The exhaust gases pass through it. Chemical reactions occur to convert harmful gases into less harmful gases.

There are two ways that they stop functioning. The material the gases pass through falls apart, or it gets coated with oil if your engine is using oil. Taking into account the amount of miles you have, and how quickly the code returns, I would guess that the material has come apart. This can create another issue.

When it comes apart, it can clog the second CC. The fact remains that if the CC in the manifold has come apart inside, the code will return consistently. The proper repair is to replace the catalyst converter.

There are aftermarket parts available that are less pricey than Nissan parts. It is against the law for a business to sell a used CC. If you would like this repaired, or just a quote, consider YourMechanic. A certified technician will be able to assist you with a further diagnoses of your Check Engine Light and help you make the necessary repairs. Good luck.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Engine not getting gas properly.

Hi, this is a common mistake people make when putting a fuel pump on. They forget to change the fuel filter. A clogged fuel filter may be what caused the pump to go bad. I would have a technician such...

Q: Why is my transaxle light flashing on and off?

In general, if your dash warning lights flash briefly right after cranking your engine and then turn off, this is normal. Each system is performing a self-check. The lights turn off after verifying that the systems are operating. If one...

Q: I have a 2003 Subaru Forester and upon starting this morning it ran fine for a minute or two. Then the Check Engine Light came on.

Too much air flowing into the intake causes the engine to lean out too much air in relation to the fuel and misfire. At an idle, the PCV valve restricts air flow, to reduce this problem. At high manifold vacuum...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...