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Q: p0455 evap.large leak on 2002 Nissan Frontier

I recently had a smoke test done on my truck.There are two codes p0455 &p0328 knock sensor.I had these repairs made.
Replaced the knock sensor,and evap.canister vent control valve.
Now the service engine is on again with the p0455 code again. I just spent $600.00 for the repairs and I still can't pass the emmision tests.What should I do next?
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hi i am a mechanic at a small dealership. we do get lots of cars from bank repo and auctions and many of them do come in with p0455 codes. many folks, when they see a check engine light they panic and automatically think the car has major issues. this is totally not the case when it comes to p0455 code.

to address the issue i would first check the gas cap, make sure its not loose or in some of my cases, it's completely missing. that would throw off the fuel system pressure and send off the DTC (diagnostic trouble code aka check engine light). if this does not seem to fix the problem, i would trace the evap line from the gas tank to canister to the engine bay. it may be confusing because from the canister to the engine bay, the evap line looks same as the metal fuel line. however, the metal line is very sturdy and hardly ever fail. the evap line leak, if it is, its most likely to occur at where the metal line ends and starts turning into preformed plasic and rubber hose connections. usually the weak preformed plastic hose will crack or snap, and the rubber hose connections will crack. do not underestimate even the smallest cracks in rubber, evap system is exremely sensitive and will detect even the minute of leaks. its ok to use electrical tape to seal the leaks for "temporary" purpose. but in the long run , the harsh gas vapors running through these lines will melt the tape and adhesive and put another hole in it. so it's better to replace the failed hoses.

now how would you know what hoses to trace? well most cars have evap system diagram on the underside of the hood. now find your PCV valve (usually on top of the valve cover). since this is most easily identifiable (at least for me it is) this would be a good starting point. from there on using the diagram you should be able to find the hoses and major components.

if you are having someone service the car and they tell you to replace all these major components such as canister and evap pump, then you should proceed with suspicion. many inexperienced mechanics will tell you to just change all these major components just because they're too lazy to search for leaks. if anything, changing out the major components should be the last resort. i do understand the searching process is a pain in the rear. but with little dedication and patience anyone can fix this problem.

combination of codes for knock sensor and evap system could very well mean that you're leaking fuel somewhere. while checking the metal evap lines from canister to the engine bay, check the fuel line too, usually they are right there together. this could also explain why the smoke test did not detect anything, since fuel, well is liquid.
Here is a very informative article about this code:
You can print it out and take it to the shop, it has info for customers, but also very technical info for the mechanic working on the vehicle.
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