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Check engine light came on

We have a 2004 Volkswagen toraeug. 1000 miles ago the check engine light came on and the dealer (I will never go back again) said the catalytic converters were shot. We were going to get second opinion but when we picked up the car the engine light was off…so 1,000 miles later it came back on. Is it a good idea for us to buy a diagnostic tester and see what the issue is ourselves then take it to a mechanic and have them fix that exact issue? Does anyone know a reliable place in Manhattan?

Check with an AutoZone or Midas, they read codes for free. Get the alphanumeric code, not just their interpretation and post here. We can help then. If you do decide to buy a code reader, the cheapest model is just as good as the high priced units for most people.

Thanks! Does the reader i can buybonline for Volkswagen/audi’s as good as what a mechanic or auto zone can do?

Anyone correct this if this isn’t true but If you just want to read the ECM codes, pretty much most will do that, I think. Just make sure it can read the newer (>2004) CAN protocol. You can get them for sub $50.
I bought an Equus Innova 3140 because it give much more info than just codes - maybe more you need, though, unless you repair your own cars.

If you own a laptop computer, all you need is a USB to OBD-2 cable and the free diagnostic program that will come with the cable…Check eBay…If the code you get translates to something like “Converter below threshold” the actual part that generated that code is the rear oxygen sensor(s). So if you are going to start replacing parts, that would be the first part to change…

so thank you for the suggestion we went to AutoZone and got the following:
P0420 Catalyst system efficiency below threshold bank 1- rich air/fuel ration, air leak near rear H02S, faulty fuel control H02S
and the same exact for bank 2.

what does this mean–what do i do? the car is also making a loud vibrating sound when driving and it one made this super loud droaning sound when i was stopped at a a light.

thanks for all your help

P0420 points to the O2 sensors and possibly the Cat but different things can cause those related codes to throw.
I’d first make sure you don’t have an exhaust leak. That vibrating and droning sound could be pointing to an exhaust heat shield that had some of its mounting points rotted out or bolts that walked out. It may just be that they are flopping around and vibrating. That isn’t all that uncommon. It could also be an exhaust leak, somewhere before one of the mufflers. A leak could throw of the readings from the o2 sensors and cause those codes to be thrown.
I’d look under the car and shake things around. Maybe one of the bolts between exhaust flanges let go. Look at the flanges that have what look like springs going around the bolts. Those joints can leaks pretty easily if there’s too much flex in the pipe.

Understand, Cats and sensors do go bad with time more often than not it points to an underlying problem that fouls the O2 sensors.

Last year, I got the dreaded cat error, and learned the system uses two sensors, one before the cat, and one after the cat. When the car is cold, they will read the same, more or less. When the car is warmed up, there should be a difference, because the cat burns up left over hydrocarbons.

If the difference is not enough to show the cat is probably working, it will give you that error. As stated, most experts agree that a high percentage of cat errors are actually much cheaper sensor failures.

Let me repeat. There is no way to check the cat, they merely look to see that the cat cleans up the air passing through it.

In my case, I drove in second gear the downhill run to Orizaba which drops one mile in 17 miles. That long a distance with the injection shut off probably allowed the cat to get crudded up.

After some weeks the cat cleaned up and the light went out.