How do I check for a vacuum leak? and fix

check the intake manifold gaskets for a vacuum leak.

P0171 indicates a lean condition. The most common cause of this is a defective mass air flow sensor. Also check if any vacuum leaks.Replace MAf sensor,if cleaning the MAF does not help.

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem,.

Thanks.

Check the IAC (Idle Air Control) valve, it is located by the Throttle Body.

have you checked for vacuum leaks?

if small leak indicated it is most likely a gas cap

Replace upper intake gaskets. They are made of metal and plastic. A common problem. Do not remove the very top plenum cover gasket.

you can use propane to find a vacuum leak take a bottle and put a rubber hose on it so you can direct it in specific spots and just let a little gas flow if you find a vacuum leak it will bring the RPM of the motor up slightly do not light the torch just let the gas come out

Check the intake manifold gaskets for a vacuum leak.

If the fuel pressure was low enough to give lean codes then u would have a loss of power that you would easily notice, you say you checked for vacuum leaks, did you use a can of carb spray or a bottle of propane? it only takes a very small leak to trigger the codes, spray carb spray around the intake area, see if the engine sound changes, this will indicate a leak,

hello i would test your( tps) throttle position sensor some part stores will test for you .goodluck