Check engine OBD2 code P0420

splash · **Posted:** Wed Mar 05, 2008 8:02 pm

Scanned my liberty 2003 with my new memoscan

Check engine OBD2 code P0420
Catalyst efficiency

Anybody have this trouble???
What to do????

Thanks

terry

Dave08 · **Joined:** Fri Nov 02, 2007 4:17 pm **Posts:** 251

Most likely either your Cat's are bad or your post cat oxygen sensor is. Mileage? Sure others can help more.

splash · **Posted:** Wed Mar 05, 2008 8:27 pm

100000 km
Oh sorry!!!!!!!!!!

60000 MILES

Dave08 · **Joined:** Fri Nov 02, 2007 4:17 pm **Posts:** 251

Well from my understanding... Either something is malfunction on the O2 sensor (including a bad connection) or the cat. converter(s) are shot. If the cats are bad, at least in the US within 100,000 miles the dealer is required to replace. (Must take care of all emission devices for 100,000 miles or like 8 yrs I think

splash · **Posted:** Wed Mar 05, 2008 8:45 pm

ok thanks!!!!
i'll call my dealer tommorow

terry

tjkj2002 · **Posted:** Wed Mar 05, 2008 9:09 pm

Quote:

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings to estimate how well the catalyst is actually doing its job of removing pollutants from the exhaust.

The upstream O2 sensor will typically show a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do much. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.

When the converter reaches about 600 degrees F, it is hot enough to start reacting with the gases in the exhaust. This is called the catalyst's "light off" temperature. The converter now starts to clean the exhaust and remove the pollutants. This causes a sudden drop in the switching activity of the downstream O2 sensor, and the sensor's output voltage levels off to an average reading of around 0.45 volts. This tells the OBD II system that the catalyst is doing its job and that everything is fine.

The OBD II catalyst monitor only runs under certain operating conditions, which typically require a combination of city and highway driving on the same drive cycle. If the catalyst monitor has run and no faults are found, the converter should be functioning properly and the vehicle should be in emissions compliance. But if the catalyst monitor runs and finds too much switching activity in the downstream O2 sensor after the converter is hot and the vehicle is being driven, it may set a P0420 code and turn on the Check Engine light.

Does this mean the converter has failed and that the vehicle is polluting? Not necessarily, but technically speaking the vehicle is NOT in emissions compliance if it has a code and the Check Engine light is on.

splash · **Posted:** Wed Jul 23, 2008 4:24 pm

Back to the top!!!

Did anybody get this problem on their liberty ???

tjkj2002 · **Posted:** Wed Jul 23, 2008 4:52 pm

splash wrote:

Back to the top!!!

Did anybody get this problem on their liberty ???

Have you checked for vacumm leaks? Air filter? Fuel filter? Spark plugs? Have the operation of the O2 sensors checked? Have the carbon cleaned out of your engine? Checked for any exhaust leaks?

NCC-1864 · **Posted:** Sat Jun 12, 2010 10:27 pm

splash wrote:

Back to the top!!!

Did anybody get this problem on their liberty ???

I have this problem ,right now (P0420)
with very poor fuel consumption 14mi/g or 20 L/110km
dealer was saying that I need new catalytic converter will cost me over $2000 CDN
looking for other solutions.
Had some rattling for a while in the muffler it's gone now
but still my CEL is on ,reminding me that I need to fix it.
so you end up with any good solutions?

RED_KJ_666 · **Posted:** Sun Jun 13, 2010 4:28 pm

Used to get it from time to time ended up being a bare wire on the 02 sensor shorting out. Or at least that fixed it lol

jeepmedic46 · **Posted:** Sun Jun 13, 2010 4:51 pm

I'm having that issue right now. Going to be having a engine diagnostic on it. I want to see if its the o2 sensors and not the catalytic converters.

MKJeepers · **Posted:** Tue Jun 04, 2013 6:47 pm

I have a 2006 Liberty and the P0420 code came on, shortly after that 3-4 days the engine lost power and was undriveable and registered a (random misfire code). The mechanic said the passenger side cat was burned up and plugged. $1300 later with a new set of cats the CE light came on again. The O2 sensors both check out fine on the scanner and are reading with in the tolerances. I assume that something else is causing the 3 passenger side cylinders to put too much unburned fuel into the exhaust. Prior to having the cats replaced I pulled all 6 spark plugs and the driver side 3 were tan colored and the 3 passenger plugs were charcoal black.

Any thoughts or ideas what might be the cause of the CE light to be triggered.

Check engine OBD2 code P0420

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