Hi guys i wait for this moment a lot.
Esterday night my ses light go on. So i check at Autozone today and they found P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Cat was changed 3 weeks ago. What can i do, and what is that mean.

I would take it back to who did the work 3 weeks ago. I would not mess with anything. If they find that someone tampered, that will be an easy out for them. They will say you did it. Just take it back. I would also take the paper work with you from who ever scanned your car.

Normally, it means the CAT convertor is not working correctly or you have a possible monitoring problem with the 02 sensor after the CAT. Was the replacement CAT a GM/OEM or aftermarket? I'd take it back to the dealer or shop where you got the CAT from and have them look at it.

It means your car wants to smoke some marijuana. J/k you need some humor your poor car has been a headache for you. take it back to who fixed it before could be a bad cat or one thats not OBDII complaint

Guys you dont understand. I bought cat online from somewhere in NewJersey.
The shop just charge me for instalation (60$) They just change 4 nuts and 2 bolts, i cant go there. I checd couple posts its looks like NON GM cat, and those dudes put some kind of simulators, so the PCM think everithing ok. The problem giving only Bank 1(controling fuel etc.) Bank 2(after cat) is nothing its just bull shit sensor, some guys just erasing code from PCM at all.
I am checking from the back and flow is fine, its just aftermaket cat(high flow).But the thing is if iam gonna clear him with scan tool he is gonna come back.

Guys you dont understand. I bought cat online from somewhere in NewJersey.
The shop just charge me for instalation (60$) They just change 4 nuts and 2 bolts, i cant go there. I checd couple posts its looks like NON GM cat, and those dudes put some kind of simulators, so the PCM think everithing ok. The problem giving only Bank 1(controling fuel etc.) Bank 2(after cat) is nothing its just bull shit sensor, some guys just erasing code from PCM at all.
I am checking from the back and flow is fine, its just aftermaket cat(high flow).But the thing is if iam gonna clear him with scan tool he is gonna come back.

OK, that explains it....somewhat. The DTC error code P0420 is not typically generated by the Bank, Position 1 sensor (before CAT). There are no Bank 2 error codes or sensors for your Impala. The 02 sensor after the CAT is Bank 1, Position 2. It sounds like you have a compatability problem with that aftermarket CAT convertor. What brand is it? It is still an inefficiency problem related to the CAT. BTW - The rear 02 sensor is not a total B.S. sensor. In conjunction with the front 02 sensor it will provide minor lean or rich trim signals for the fuel injection system.

If all else fails you may just need a better quality rear 02 sensor simulator so you can run that that specific CAT. You can reset P0420 all day long but it will probably come back so I agree. Good luck!

A high flow cat will still do what the cat is supposed to, but it will restrict the exhaust flow much less...

The Impala PCM is programmed to expect a certain ''flow'' at the rear O2 sensor. If it sees more than the stock flow, it throws a PCM code. An O2 sim does just what it says...it simulates the signal from the rear O2 under stock flow conditions so the PCM doesn''t set a code... you can also get a custom PCM tune to disable the rear O2 codes altogether...

Rear O2 is only used to detect the functional level of the cat, not to set the performance or air/fuel mixture of the motor...

Ok guys, my head is upside down.
Here what i got on recipt from Autozone.

"Troubleshooting P0420
The PCM has determined that the catalyst system efficiency for bank 1 is below threshold for the current engine operating conditions.(bank1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)"

So where is bank 1 and 2. Close to the spark plugs what bank is it? And where iam having problems on the ubend or that which close to sparks.

Ok guys, my head is upside down.
Here what i got on recipt from Autozone.

"Troubleshooting P0420
The PCM has determined that the catalyst system efficiency for bank 1 is below threshold for the current engine operating conditions.(bank1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)"

So where is bank 1 and 2. Close to the spark plugs what bank is it? And where iam having problems on the ubend or that which close to sparks.

You cannot go by Autozone and their generic descriptions. It depends on what is programmed into the odb-ii scanner. I go by GM's Mode 6 DTC error codes and what is specified by the GM service manual. Normally when you have Autozone scan the error code will probably be correct but their description will send you on a wild goose chase.

There are no applicable Bank 2 codes for exhaust or fuel injection for your engine. Cars with true dual exhaust running multiple 02 sensors and dual CATs will generate Bank 2 codes.

On the 02 simulators they are not intended for street operation. Off-road only.

The 02 sensor does not detect exhaust flow per se. It looks at oxygen content. The OBD II system compares the oxygen level readings of the O2 sensors before and after the converter to see if the converter is reducing the pollutants in the exhaust. If it sees little or no change in oxygen level readings, it means the converter is not working properly.

Did you say you had an 02 simulator after the CAT installed?

No iam saying i just order and gonna have it on tuesday. I just asking where to put? On the first or on that which on u bend. And you sayd something about off road, what you mean. Can i drive with it in the city, and gonna be still happy or what?

the o2 sim goes down by the ubend down under the car after the cat

BNAYLOR3400 sayd "On the 02 simulators they are not intended for street operation". What you think its gonna work in the city, or its something different?

BNAYLOR3400 sayd "On the 02 simulators they are not intended for street operation". What you think its gonna work in the city, or its something different?

Your car will probably work fine due to your situation with the aftermarket CAT. All I said was the simulators were intended for off-road use and not for a grocery getter or daily commuter. And not to circumvent an emissions problem. Do you race your Impala? The only people I know using them are by buddies at the drag strip. I have a Magnaflow Hi-Flow CAT and have no problems with ever getting a P0420 code so I don't need one.

The only time I'd worry about it is when it is time for the emissions inspection, especially if you live in a green state.

Thank you guys. Another question how i can check MAF sensor? Is there suposetobe any omhs on 3 legs. Cause i check it and i have some on 2 legs only.(engine is off). I really cant find the problem(No power, hard shift).Tommorow i wanna spend whole day on checking some stuff. What else i should check?
Thank you

Thank you guys. Another question how i can check MAF sensor? Is there suposetobe any omhs on 3 legs. Cause i check it and i have some on 2 legs only.(engine is off). I really cant find the problem(No power, hard shift).Tommorow i wanna spend whole day on checking some stuff. What else i should check?
Thank you

All you can check on a MAF sensor is the PCM 5 volt reference signal at the electrical plug. The output going back to the PCM is a frequency related signal depending on airflow. Technically, you'll need an oscilloscope to check that signal. Checking ohms is an exercise in futility on a MAF sensor due to the internal electronic components.

What was the specific MAF DTC error code, if any? What engine do you have?

I have 3.4 and no codes. Nobpdy cant find the problem. I checd alredy everithing and still no power an hard upshift?

I have 3.4 and no codes. Nobpdy cant find the problem. I checd alredy everithing and still no power an hard upshift?

Head Scratcher! Just a suggestion but have you started looking at an internal engine problem?

3.4 is known to have flaky intake manifold gaskets. Both UIM-Throttle Body and LIM. Are you losing any coolant?

How about a compression test on each cylinder? Minimum is around 105 psi. Normally good cylinder compression is around 125 - 150 psi. Test is run with all spark plugs removed and with fresh engine oil.

Other possibility is your torque convertor clutch (TCC) not working properly. Maybe it is staying locked up?

What brand spark plug, heat range, and ignition wires are you using?

No iam not loosing any coolant. I think owner before fix gasket. About spark plug and wires acdelco (brandnew). (fuel and air filter new as well, pcv, cat)
The symptoms is like engine cant breath, like cloged cat. But i put Direct Fit, maybe aftermarket one but this dosent metter. Only thing i newer changed its o2 sensors.
How i can check injectors for carbon. Cause when car is hot i feel little vibration(onthe neutral), dosent idlle smooth (no codes of misfire)

No iam not loosing any coolant. I think owner before fix gasket. About spark plug and wires acdelco (brandnew). (fuel and air filter new as well, pcv, cat)
The symptoms is like engine cant breath, like cloged cat. But i put Direct Fit, maybe aftermarket one but this dosent metter. Only thing i newer changed its o2 sensors.
How i can check injectors for carbon. Cause when car is hot i feel little vibration(onthe neutral), dosent idlle smooth (no codes of misfire)

Ok I guess we can rule that out.

Have you tried running any fuel injector cleaner like Lucas through the gas tank?

02 sensor is a possibiity but I doubt it. When accelerating or when power is needed the PCM is in open loop mode which means the 02 sensors are out of the picture (inhibited). They are used at idle after engine warms up to proper operating temperature, cruising, etc. Closed loop mode only.

On the 3.4 you can manually clean injectors by removing the throttle body and upper intake manifold plus anthing else in the way like electrical connecors and vacuum lines. Then you remove each fuel injector rail with the injectors intact. You can leave the fuel supply pressure and return lines connected to the rail or disconnect them. However, once injector is pulled you should put in new o-rings.

Have you checked out the EGR valve and tried cleaning it and make sure passages from exhaust to UIM are not carboned up. Make sure EGR spring loaded pintle is clean and moves freely. Spray pintle with WD40. An EGR valve stuck open could cause loss of power and rough idle. Doesn't always give an error code or SES light either.

Good luck!

I tryed Two tank with fuel injector treatment. What about EGR i didnt know that, where is location of that bastard?
Thank you. Your help is apreciated.

I tryed Two tank with fuel injector treatment. What about EGR i didnt know that, where is location of that bastard?
Thank you. Your help is apreciated.

You're welcome. Important thing is not to give up.

The EGR valve is located between the throttle body and the ignition coil modules. It is that cylinder looking object sitting upright. When called upon it passes hot exhaust gases back into the intake to help reduce certain emissions. There are 2 10mm bolts that hold it to the flex exhaust flange and 2 10mm bolts that hold it to the upper intake manifold. There is one electrical connector. Be careful with it because replacement is expensive. Good luck!

Thank you. I am gonna check EGR tommorow.
I have one more thing never touchen, its a radiator. Sinse i boght this car, i never flush it. Can it be some heating issue. My gage shows inthe midde between 140-260. Manual says its normal. Its just, when engine is hot you can feel vibration better than on cold.

Hi BNAYLOR3400, today i checkd the EGR valve and its fine. What else it could be?

Hi BNAYLOR3400, today i checkd the EGR valve and its fine. What else it could be?

Good question. :dunno:

Final suggestion, run a vacuum test if you haven't already done so with an automotive vacuum gauge (0 - 30 hg). Acceptable engine vacuum is nothing less than 15 hg but more like around 18 - 20 hg when good. You can tee off of any intake manifold vacuum line. If your power is that bad maybe the test will show something.

Also, take the engine rpms up to 2500 rpms and hold steady when running the test. See if vacuum slowly drops off below 15 hg or towards zero. It is does then there is a problem possibly a restricted exhaust even though you changed the CAT out. Good luck!

Thank you. I will answer Satturday.