P0420 check engine light: read if you've had this

[tdelker]

Yes, disconnecting the battery will clear the code. You should read cardoc's excellent post on diagnosing P0420. Just search...

[rcolson29]

Thanks. I saw Cardoc's post and video on PO420 -- good information. I'm hoping addressing a few items will correct my car's issue. It has run rough on a couple occassions during extended wet / moist air weather, so perhaps an underlying issue is affecting overall engine performance enough to cause the inefficiency code to surface.

[cardoc]

Low idle may be a key to the issue. Too much fuel getting shot into the combustion chamber or a crappy shot from a dirty injector. Idle control valve sticking. Dirty throttle body. AF sensor is getting slow and the computer is adding too much fuel.

This is another where data is needed to isolate the issue.

On gasoline types: I have a customer who gets Wal Mart gift card for the holidays but he doesn't shop there. He would use it for gasoline on occasion just to use the card up. He drives a lot. Within a month's time, he came to visit me because of a P0420 on his Hyundai. It had less than 35k miles and he knew I would tell him straight up what the deal was. The only thing I could find at issue was the gasoline. We ran a test. He drove with the light on through to his next fill up when he went back to his regular brand, Shell. After driving about 50 miles, the light goes out. The code is still stored in memory, just no light and not active. A couple tanks later and he goes back to Wal Mart and within 25 miles, the light comes on and its a P0420. I left it on. He filled back up with Shell after it ran low and again, the light went out after driving about 50 miles.

Sometimes its just too simple.

[signage]

Originally Posted by cardoc

Low idle may be a key to the issue. Too much fuel getting shot into the combustion chamber or a crappy shot from a dirty injector. Idle control valve sticking. Dirty throttle body. AF sensor is getting slow and the computer is adding too much fuel.

This is another where data is needed to isolate the issue.

On gasoline types: I have a customer who gets Wal Mart gift card for the holidays but he doesn't shop there. He would use it for gasoline on occasion just to use the card up. He drives a lot. Within a month's time, he came to visit me because of a P0420 on his Hyundai. It had less than 35k miles and he knew I would tell him straight up what the deal was. The only thing I could find at issue was the gasoline. We ran a test. He drove with the light on through to his next fill up when he went back to his regular brand, Shell. After driving about 50 miles, the light goes out. The code is still stored in memory, just no light and not active. A couple tanks later and he goes back to Wal Mart and within 25 miles, the light comes on and its a P0420. I left it on. He filled back up with Shell after it ran low and again, the light went out after driving about 50 miles.

Sometimes its just too simple.

What WalMart gas has CEL light molecules in it

[rcolson29]

I'm picking up the PCV valve today from O-Reilys, but I did throw a new air filter in last night. I disconnected the battery to clear the current code and noticed it took a few seconds more than mormal for it to start. Once started the idle speed was extremely low -- to the point I thought the engine was going to kill. It ran fine at intermediate and higher speeds, but each time I pulled up to a stop the idle speed was very low.

We get gas at a variety of stations, so I'm not sure fuel is the issue or not. From prior posts it sounds like I should give a can of sea-foam a try with the next fill up..

[cardoc]

Check your battery cable connections again and check the alternator output.

[rcolson29]

Cleaned the battery connectors (had battery replaced last fall). Alternator output is at 14.1.

The low idle issue seems to dissapear after driving the car for a while, but the check engine light came back on after driving about 30 miles.

It's interesting that the very low idle only occurs after disconnecting / reconnecting the battery. It's like the system is recalibrating before it resumes normal operation.

Mileage seems to be the same as it's always been.

[subaruman49]

Hey Kristi......ever thought of moving to New England ? We could sure use a Master technician like you !!!!!

I'd drive few hours to come to you!!

[Fibber2]

I think some of you might benefit from reading thru this. It might help you understand the complexities & interactions a bit better.

An Engineers Perspective on the P0420 Catalytic Efficiency code

[rcolson29]

Thanks Fibber2 - great article even for a novice like myself. It makes sense now why the car performs the way it does after the battery is disconnected.