I have a 1998 vw new beetle. The check engine light came on and not sure why. The car seems to be running fine. Last week we replaced the tensioner pulley and a new belt. Now I have the engine light on. Any ideas?

you definitely need to replace the belt and water pump and sometimes the tensioner. i would recommend a high mileage belt and a water pump with a metal impeller.
failure to replace the belt immediately can cause severe engine damage.
believe it or not there is a youtube video that shows how to do the belt on a new beetle.

The blown head gasket problem that you repaired may have damaged the O2 sensor. A 2004 probably needs one anyway

P0171 is LEAN and P0303 is misfire number 3 cyl. Its time to stop guessing and pay for a correct diagnosis. Find a shop that works on VW and has the tools to work on it. DO NOT put those Bosch Platinum Spark plugs in that Ottozone tries to sell everybody.

I had that problem for the longest time. I had to clean the Mass Air Flow Sensor. Very easy to get to. Go to any auto parts store and ask them for the MAFS Spray. If cleaning it doesn't work, you may need to replace the MAFS. Once again easy to replace. Make sure you put it back nice and tight.....

Could be a lot of things, take it to autozone. More than likely they can scan the code for you, they can't tell you exactly what is wrong but they can steer you in the right direction. Give them a call to see if they are able to scan the codes on your VW. Good luck.

P0411 Secondary Air Injection System Incorrect Flow Detected
The 4mm inside diameter vaccuum hose gets pinched between the lower intake manifold and the secondary air pump housing, flattening it over time.
Here's a link to the steps to fix yourself if you are mechanically inclined...
http://forums.vwvortex.com/zerothread?id=1995162

P1128 Long Term Fuel Trim Is Too Lean
It is very likely this code is coming up because of the vacuum hose as well.

the heater core could be leaking that explains the fog but i would have the water pump checked

BTW - if you have less than 100,000miles you might be able to have your cat replaced under warranty by the dealer. In Canada it's 160,000km.

Sounds like you have a gremlin running around under the hood, just kidding. The check engine light was probably caused by a failing sensor, or it could have been a loss of oil pressure due to the old oil. If the problem never returns then I think it is safe to say it was a loss of oil pressure, but if it is a failing sensor this will occur again. When and if it does happen again do not reset the computer, take the car to an autozone(they diagnose check engine lights for free) if you have one around you. The diagnostic will tell you exactly what sensor has caused this, but if you reset the computer ther is nothing to diagnose. And a dealer or shop can charge 60-90 dollars for the diagnsis. Based on my expirence if it is a sensor it is probably the MAP(Manifold Absolute Pressure Sensor) this sensor is probably the only one that will completly shut down the car, most other sensors will fail and your check engine light comes on but the car stays running.

Hope this helps

check your mass air flow sensor , it take over fuel pump control after the engine starts