my check engine light is on and i have changed the down stream o2 sensor?

 

did you check what codes you are throwing? Truck off, hold down the trip meter and then turn the truck to acc. There will be a code displayed at the bottom where the mileage is displayed. Might be easier to help diagnose the problem.

 

Do yourself a better favor and take the truck to a local auto parts store and tell them the CEL is on. Let them connect their ODBII scanner to the truck and tell you what codes are on. Then bring them back here and post them so we can help you out.

Doing the procedure listed above will work but will not give all the DTC codes that might be logged.

 

If you have poor gas mileage changing the downstream sensor won't do anything. The upstream sensor is what tells the PCM weather your truck is running rich, lean etc. Like stated above get the codes read, but i would guarantee your problem is the upstream sensor and maybe a plugged CAT.

 

Code hr3532

 

chat is code 3555?

 

hr3532 is how man hours the truck has logged.

Go back and re-read post 3 please

 

p0420 is the code

 

P0420 Catalyst Efficiency Below Threshold (Bank 1)

 

That could be a bad front O2, a bad cat or a leak somewhere in front of the O2 and cat. Check the exhaust manifold bolts on that side and make sure none are broken or loose