I have serv eng light on with code # PO172 , PO128 , and PO442 , and I replaced fuel tank cap today , light still on ,and I need to get inspection , but I am out of work and can't afford expensive repair at dealership , and I suspect I have a faulty Oxygen sensor , car runs fine , no problems at all noticed other than light on , 03' Chevy Trailblazer 2w.dr. 6 cyl. , any sugestions ?

this relates to a slight leak of fuel vapour to atmosphere from the EVAP system , to check undo and refit gas tank cap and clear code , then drive for a day and rescan for code , if reappears then the EVAP system will need inspecting for leakage

that is the evap sensor failure. it can be multiple things. gas cap allows it to come on sometimes. delete the cel and see if it comes back on. if it does it can be the evap sensor located on top of fuel tank, thereis a charcoal filter located in there somewhere that filters the gas fumes before it exits into the environment. with the work involved to drop the gas tank and everything, it is easier to replace all of it once you have it apart. that way you dont just change the sensor and put it all back together and it be something else.

The Purge Valve is normally on the Top if the Gas Tank, The Evap Canister is forward, follow the GREEN EVAP line to the Canister. There is a chance that it could be near the fuel tank but you never gave the year/make/model for a specific vehicle but this will get you there...

P0442 code means that the control module has detected a small leak in the Evaporative Emission (EVAP). The loose fuel tank filler cap is the most common cause that triggers the P0442 code.

Code P0445 indicates that there is improper purge flow in the EVAP system caused by a shorted purge valve.

Make sure Fuel tank cap is tightened well, then have it tested at Autozone, etc... If ok, then go back to inspection station.

evap system leak small most of the time this code comes from forgetting or properly replace their gas cap--others are gas cap not sealing replace cap-or a small leak in the fuel vapor lines -check cap and reset code if comes back check seal arund cap repair or replace if damaged -if good inspect the fuel lines and hoses for small cracks or damage repair or replace if necessary

Hello! Those three codes commonly indicate the followi possibilities:

The gas cap was not sufficiently tightenend ater fueling, a bad gas-cap rubber o-ring, the vehicle had the key in the 'on', or engine running, or the tank was filed all of the way up the neck.

Any one of the abope causes will set all three codes.

An excellent read on DTC P0128 can be found here:

http://www.hotrodlane.cc/New%20LS%20Links%20Page/LS%20trouble%20codes/DTC%20P0128.htm

P0172 is a rich fuel condition in bank 1. Have you had any work done lately? A lot of time a leaking fuel pressure regulator will trip this code.

The evaporative emission system is designed to prevent fuel vapors from being released into the atmosphere. The vapors are absorbed and stored by a fuel vapor (evap) charcoal cannister until certain engine conditions are met, then the vapors are purged and burned in the combustion process. I would lean toward a faulty purge solenoid based on the condition that you describe. I would also look at a faulty gas cap. The gas cap is an integral part of the evap system and will cause these kind of problems if faulty and not venting properly. Hope this helps and that you find a resolution to your problem.

You likely won't notice any drivability problems. P0442 indicates a fuel vapor leak in the EVAP control system. It means a very small leak has been detected. In fact, the leak can be from a hole as small as 0.04" in diameter. 
The (EVAP) emission control system prevents the escape of fuel vapors from a vehicle's fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage.
Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine.

With a P0442, the most common repair is to Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back. Otherwise, replace the gas cap, or Inspect the EVAP system for cuts/holes in tubes/hoses.

Good luck and hope this helps

P0442 EVAP System Small Leak (0.040") Detected


EVAP fuel tank or canister vapor hoses leaking or damaged
EVAP system component leaking, damaged or has failed
Fuel tank cap is loose, or the cap release pressure is incorrect
Leak detection pump damaged or leaking

You can try the gas cap first, if that does not help you will need to take it to a repair shop, Small leaks are very hard to find. If you want to know what turns on the light here it is.

Cold engine startup (ECT sensor from 40-90�F and within 10�F of the BTS input at startup), fuel level more than 1/2 full, engine running, EVAP leak test "on", and the PCM detected a leak greater than 0.040� but less than 0.080" somewhere in the system.
This code sets in 2 Trip(s).