PO016, PO300, PO420, PO442 codes - 2003 GMC Envoy

Dont change things if udont know solution..its just guessin. A flashin enginelite means danger. It often means the catylitic converter is bad. But hav adata stream run at a privateshop to get values to target issue. Codes are jyst approximate n general

These codes are unrelated. 442 indicates a leak in the evap system or a gas cap that is not sealing properly. 300 indicates bad ignition module, crankshaft sensor, or worn out plug wires. Are there any symptoms?

Failed Ignition Coils

Resulting cylinder misfires over time
have ruined the exhaust converters (PO420 Bank #1)

PO442-small leak in Evap System

It's all on the internet ,if you Google your codes
& study OBD2 Systems

Type: OBD PO 442

On a 2005 vehicle-- All-- the ignition coils will be bad after
7 or 8 years

At 100,000 miles the front Oxygen Sensors need to be replaced

all the codes you have listed are concerning the exhaust/emissions and random misfire , check all air intake hoses for leaks/wear etc ,check condition of spark plugs renew if in doubt ,take it from there

po300 is a random misfire code(could be egr passages blocked under upper pleinum;plug wires;plugs or coils.PO420 is catalatic converter below threshold(converter has to be replaced)

http://www.obd-codes.com you can check your codes here and some info on these codes

The p0300 code is a random missfire code sometimes caused by bad spark plugs and wires, if you have not done a tune up, it's time for one. now, the p0420 code is for catalyst efficiency below threshold, basically, your catalytic converter is bad and may need to be replaced. Sometimes repairing the random missfire can eventually clear the p0420 code. your car is not burning fuel properly, therefore the catalytic converter is getting dirty. Also, if you notice blue smoke coming out the tail pipe, then that's the cause for a failing catalytic converter as oil is getting to the combustion chamber and being burned off through the exhaust.

Mulitiple cylinder miss fire,May be the coil boots and plugs,will fix your problem,and,or fuel injection cleaner with a couple of bottles of HEET,a water dryer to put in the fuel tank.

I have tow of the three requested codes. po300 is misfire detected at multiple cylinders. See po301-306 represent the cylinder # misfire. So 301 is cylinder #1 misfire.

po420 is Catalytic Converter Efficiency Failure

okay po442 isevap leak check gas cap amd right front tower in engine check evap line for broken hose,po131 bad o2 sensor top sensor on rear exhaust manifold,po420 is bad catalyic converter but could be o2 sensor.i would replace o2 first reset eng lingt and go from there, hope this helps good luck.