I have a DHS with 160000 miles. I keep getting this code. I've replaced the CAT and the three sensors. I've put on a new connector. The strange part is. I only get it while driving in and around town. I don't get it on long trips. I had a smell from the exhaust. I had this code before and after I've changed the CAT. Any ideas???

 

This trouble code will not show up in highway driving because the PCM performs the test required to set it when the engine is hot and at idle. The fact that the trouble code doesn't trip with highway driving is normal.

Please provide additional information:

Are there any additional trouble codes?

Was the cat replaced with an original equipment catalytic converter?

Are the oxygen sensors installed correctly?

Is the entire exhaust system including the secondary air system intact?

Is a high sulfur fuel being used?

What is the octane rating of the fuel?

Is the car certified for California emissions and operated with fuel from another state?

 

I live in Maryland. No other codes. I didn't use an OEM CAT. All oxygen sensors are new and the exhaust system is entact. As far as the sulfur, not sure if it's high or not.

 

The P0420 code indicates that the catalytic converter is not storing enough oxygen. The PCM controls the rich to lean fuel mixture and compares the results between oxygen sensor 1 in front of the catalytic converter to oxygen sensor 2 behind the converter while looking for a time delay between the sensors status change. A lack of a time delay indicates that the catalytic converter is not storing the enough oxygen to burn the excess fuel. The oxygen for the catalytic converter comes from the secondary air supply system and additives in the fuel. It is that lack of a time delay that triggers the code and the MIL.

Since you have already checked the trouble codes, exhaust system, and secondary air injection system, I can only think of a few possibilities that could be causing the problem:

1. Check the oxygen sensor fuse in the under hood fuse block.

2. Check if the vehicle was designed for low sulfur fuel used in many of the western states. The vehicle doesn't have to originate in California to be designed for California emissions.

Look at the underside of the hood near the driver to see if California emissions are indicated on the label.

A California emissions car designed up to 2007 will have a 'NC1' RPO code on the service parts identification label which is located in the trunk, probably on the underside of the spare tire cover. You can also find your RPO codes by entering your VIN into one of the online VIN to RPO code converters. Here is a link to one: http://www.compnine.com/vid.php

3. Try changing to a high quality, 91-octane fuel for a couple of tanks. Use almost all of the fuel in the tank before switching brands.

4. Have a shop with a Tech II trouble shoot the oxygen sensors, rich to lean fuel mixture controls, wiring, connectors, and PCM.

5. Bring the car back to the shop that installed the catalytic converter because the converter is not working.

 

Thanks for all the information. I haven't checked the secondary air injection. I've wondered if the pump is working. I know where it's on the car. How would I check to see if it's working? I know it's to provide air doing cold starts to speed up engime warm up. I'll check to see what the RPO is. I purchased the car used.

 

The secondary air injection air system does not assist with cold starts. The system provides fresh filtered air into the exhaust stream for the catalytic converter. The system consists of an air pump, vacuum solenoid, air shut off valves, check valve, and plumbing to the exhaust system.

 

AIR system normally will set P0410, pump is located behind drivers side front behind bumper. It will hum / buzz approx 2 min, right after engine is started. Is it a GM cat? There is no air pipe to cat. Do you have access to scanner, to monitor the 3 O2 sensors? The PCM/ECM looks at data from the 2 front O2 sensor and the action of the rear. The action of the rear one causes code to set. Are any exhaust leaks?

 

There isn't aren't leaks that I can normally hear. The only break was installing the CAT. It's not a GM CAT and it doesn't have an air pipe. The OEM one didn't either. I can see the plumbing running around behind the LF tire. I'll check to see were it connects to the exhaust system. I do remember hearing the sound of air hissing right after start up when the engine is idling higher. Once the idle comes down. I don't hear it.

I don't have a scanner just a code reader and one that can check the emisson. I don't know if I can monitor sensors with it.

 

The plumbing for the secondary air system supplies air directly into both exhaust manifolds not into the cat itself.

You probably have a bad cat, but it is important to verify that the conditions required for the catalytic converter to operate properly are present.

The oxygen for the catalytic converter is supplied from two sources, additives in the fuel provide oxygen at normal operation and the secondary air system provides additional oxygen at startup. The P0420 test will start when the coolant temperature reaches 160 F while the secondary air system is still operating. If excess oxygen is not available in the exhaust stream, the catalytic converter will not be able to store and release it when required, which will result in the bank 2 oxygen sensor acting the same as the bank 1 sensors and trigger the P0420 trouble code.

A problem with the secondary air system will probably trigger P0410 as slowroll mentioned or another code such as P0412 or P0418. However it is possible for it to cause the P0420 code without triggering the other codes. I would check the air pump filter and secondary air plumbing for leaks using soapy water, but I wouldn't worry about inspecting the check valve or solenoids because the other codes did not set.

Also the fact that P0410 and other codes did not set indicates that the oxygen sensors are probably operating correctly.

Don't forget to verify that the car is not a NC1 - California emissions car before convicting the cat.

 

My guess is the converter is mabe half the size it should be. Non - OEM converters many times have a reduced capacity compared to OEM - they are cheaper but not equal to. You may need different brand or OEM (GM in this case) converter. You really need a scanner to find out whats going on. If you paid someone to fix P0420 and they sold you the converter as a fix, take it back and let them figure it out.