Yup. Elm 327 is good for the price, which has dropped since I bought one. I also have a GM Tech 2, the same as GM dealership mechanics use that will do a little more. But the Elm 327 will get the job done.

 

ok what we found out today is the ltft was up and found a vacuum leak around the egr valve..there is only 1 gasket on the egr and the pipe going to exhaust and the pipe going to the intake..under the egr someplace there is a vacuum leak,enough you can hear..stft was 20 % when we sprayed the spot the stft dropped to 6%..so im tring to figure out where it is coming from..

 

Ah ha! Sounds like you have one of the most common, yet elusive vacuum leaks there are. Check for small cracks in the tube from the EGR to the intake.

 

yea its under it..so much yuo can hear it now..and there was 2 bolts missing from the manifold,1 on the very end near the egr also..

 

ok so now,no leak and after 30 miles or so I get P0420 - Catalyst System Efficiency Below Threshold Bank 1
..now what.
Read more: P0420 Catalyst System Efficiency Below Threshold Bank 1 OBDII Engine Light Trouble Code | Engine-Codes.com

 

The article you uploaded is pretty comprehensive. Some scan tools have a catalyst test function to tell whether the Cat is bad, but I would be most suspicious of an exhaust leak.

 

Have you recently replaced your Cat? Maybe installed an Aftermarket Cat? P0420 refers to the After Cat O2 sensor. That sensor checks the temp of the exhaust gasses. Compared to the before Cat O2. If you have an Aftermarket Cat, it will cause that P0420 Code.

 

never touched the cat so I don't know...I don't hear an exhaust leak, I had new mufflers put on last yr..why wouldn't the code come up a while ago,was the 171 code mask it? so need to scan for O2 codes?

 

How many miles are on the car? I assume it's up there if you've had to replace the mufflers. It may be time to replace or at least clean out the Cat. Does it smoke at all?

 

If you never changed your Cat then it could be a bad after Cat sensor or the Cat is going. More than likely it's the sensor. Otherwise you should get a code for the before Cat sensor.