Evap code..ch eng lite on...repl evap valve rear of car outside tank..code stays off for couple weeks to couple months then come back on not gas cap problem

A very small leak is indicated by the computer which is usually the gas cap. When you change the cap make sure that the threads in the tank are clean. Spray a little wd-40 on them.

gas cap code p0449 ??? This is not a gas cap code ! DTC P0449: Evaporative Emission (EVAP) Vent Solenoid Control Circuit.. your best bet , take it to a qualified chevy repair shop .


Wells EVAP Training with Sgt Tech P0440 P0441 P0442 etc

Check for applicable trouble codes, see if anything related to evap system? Your emissions system probably pulls a small vacuum on the gas tank and evap system to check for leaks, If it doesn't pass the test, you will get an evap code or gas cap lite? The place that replaced exhaust system can they check for trouble codes? Anything?
To check for codes, yourself, at the least, you need an obd 2 code reader.
I don't know if exhaust repair caused a problem or just coincidence?

code p1456 refers to evaporative emission (EVAP) canister purge system(fuel tank system) -leak detected===causes--hose-fuel tank/pressure sensor--fuel filler cap--EVAP valve /bypass solenoid--EVAP two way valve---EVAP canister/vent valve

There is also a second reference to the code which is EVAP system leakage ====filler cap--EVAP canister purge valve--hoses---EVAP canister shut off valve---ECM programming---ECM

Check with the service dealer that the filler caps are interchangeable . It may be coincidence that the problem started after you changed caps or it may have shown a failure in the making

weak power /or poor ground to evap control relay (wiring) common.. also if purge valve stuck open it will not leak externally but let fuel vapors pass back and forth flagging codes..(which it won't due when tank is more full and vapors travel only one direction as designed) good luck

This problem is out side the gas tank , has nothing to do with the fuel pump or fuel level sending unit inside the gas tank .DTC P0446 EVAP Vent Solenoid Valve Control System . Do you know anything about automotive evaporative emission system ?
Wells EVAP Training with Sgt Tech P0440 P0441 P0442 etc

This DTC tests the evaporative emission (EVAP) system for a restricted or blocked EVAP vent path. The control module commands the EVAP canister purge solenoid Open and the EVAP canister vent solenoid Closed. This allows vacuum to be applied to the EVAP system. Once a calibrated vacuum level has been reached, the control module commands the EVAP canister purge solenoid Closed and the EVAP canister vent solenoid Open. The control module monitors the fuel tank pressure (FTP) sensor for a decrease in vacuum. If the vacuum does not decrease to near 0 inches H2O in a calibrated time, this DTC sets.

This probably means:

• P1456: Evaporative Emission Control System Leak Detected (Fuel Tank System)
• Usually this is because your gas cap was not fully tightened. Be sure to turn it until you hear a click.
• After you tighten the cap, it might take a couple days of driving before the light goes-out.
• Be careful to tighten your gas cap regularly, because otherwise it puts a heavy strain on your emission system.

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Replacing the EGR valve, its actuator, or your DPFE sensor could all be at fault. I would probably start with the valve itself since it's an EGR leak code. Your gas cap was a wasted repair. The gas cap would cause a P0441 or 445 evap code, but not an EGR code.

you were told your vehicle has small/large evap leak detected? The leak could be coming from any number of components. Fuel tank seal, evap purge valve, check valve. The best way is to find the leak with a smoke machine. It is designed to find leaks in the evap system. There is a port with a green cap in the engine compartment. This is where the hose from the machine attaches to. Turn machine on. Where ever you see smoke, this is where the leak is.

HI
can you tell us the NO. of DTC when you check it by diagnoses scan tool
that is very helpful to fix the problem
so generally my be the problem in fuel tank vent pipe or the ventilation valve
that work with the (EVAP) evaporate emission control system

give us more information to help you
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