PO442 got me po'd


On my '99 Dodge Ram 3500 van the check engine light keeps coming on. I bought it used last year and on the morning of our first big trip the light came on. $150 later I learned: the OBD2 code was PO442, the vehicle did NOT fail the ‘smoke test’, but the technician was not going to mess with the extra stuff added when the van was converted to a camper (fresh, gray & black water tanks, etc.) to further diagnose the problem.

I got a low $ code scanner so I can kill the check engine light if the code is PO442. But I’d like to figure out how to keep the light off. I’ve replaced the fuel filler cap already. What else might the issue be, given that the beastie does not fail the smoke test? And is it something an amateur could fix? (btw, any reference to learning how exactly the computer learns there’s a small evaporative emissions leak?)

Thanks for your thoughts.



To learn more about that system, go to: www.autoshop101.com. Click on Technical Articles. Then, on Engine Performance OBDII. Then, 62 Emissions #8 Evaporative Control.pdf. The acticle should answer many of your questions.


Black Tape


Thanks immensely for the reference. I guess I’m smarter now. What I surmise is that the trigger is insufficient pressure in the system - such as when a filler cap doesn’t make an adequate seal. So if the van passed the smoke test, I can conclude there are no system leaks in the engine compartment. The only place left is under the van - fuel tank and connections. About all I could reasonably do is try another filler cap. Or black tape.



Here is a link that may help. It could just be a bad gas cap. But complaining that your “smoke test” passed but he won’t give you an inspection sticker is a moot point. Nowadays, most states REQUIRE a clean OBD-II reading before passing the smog test. Your code is telling them your truck is leaking hydrocarbons into the air, so… you rightly fail the emissions certification.

(So “black tape” doesn’t work anymore either, since they have to plug the computer up.)


I suspect that you are more or less on your own on this one. P0442 should mean that the pressure in the evaporative emissions control system is different than it should be. But since the workings of ECM computers are proprietary, you probably can’t know for sure what is being checked for what conditions or how it is being checked.

Some vehicles seem to throw P0442 codes more or less at random and my impression is that no one – dealers included – has the slightest idea what to do about it.

For lack of a better idea, if you can’t find anyone local who can fix evaporative fuel control hardware, I’d read everything I could find; buy a cheap OBD2 scanner and maybe a voltmeter; grab a mess of vacuum/fuel hose, clamps, plugs, etc; and start working my way away from the Fuel Vapor Pressure sensor (or whatever its equivalent is) trying to identify the faulty component/leaky hose that is causing the problem.

Here’s another link that might help http://www.obd-codes.com/trouble_codes/generic/p0442-evaporative-emission-control-system-leak-detected-small.php