Our check engine light has come on numerous times in the last couple of years. Each time we had it checked the technicians said it was the emissions system probably caused by not closing the gas cap adequately. They then reset it. A couple months ago when we took it in the shop told us it needed to have the emissions evaporative canister replaced to the tune of $800+. At the time, we decided to just leave it since we had no other signs that we had a problem, i.e. car still started immediately, got good gas mileage and didn’t seem to have any other noticeable issues. We had to have our emissions test this month and knew that it would not pass with the check engine light. We went to a different shop, they diagnosed it and told us that we needed to have the rollover valve and hoses replaced. We were able to get help from the state for the repair which only cost $600+. My question is why the difference in diagnostic?
Unless a smoke test was performed to determine what’s causing the EVAP leak, they;re just guessing.
Tester
I’ve had the same light on for most of a year - performance is fine, but it throws me that code. The only symptom has been slow starts if I start the car several times (like driving, parking, and starting up again).
My code if PO420 - something in the air system, like an O2 sensor or a catalytic converter. Those would cost hundreds to replace, and it’s largely guesswork. But I haven’t had this smoke test done.
I did test for a hose leak though - get a propane torch, and spray the gas (NOT on fire) over the air hoses in your engine while you’re running the engine in the driveway. If it changes the noise of the car, there’s your leak, and the fix may be easy.
OP, the fuel system is designed to be air tight. Like a balloon. And just like a balloon it can leak in a lot of places. If you had a leaky balloon you could hold it underwater looking for bubbles to find the location, but that’s not possible with a car. Either each place your car’s fuel system could leak has to be inspected, or as mentioned above, the fuel system has to be filled w/ smoke and look for where the smoke escapes. Common leak points are the canister’s purge & vent valves and the valves and connections and hoses at the top of the fuel tank. And the gas cap of course. Given all the places a leak could develop it’s a wonder it works at all 
@abe.clabby_151869 … p0420 indicates a problem w/the catylitic coverter system, but not necessarily a bad cat.
I have had a small evap leak code that came up after the last emissions check, for me it could be a few things, but I have a mechanic bud who helps me from his home, and feel confident with his professional expertise the proper solution will be found. Have you tried the easiest solution, replace the gas cap? Worked for me the first time, was good for a few years, this time something else as new gas cap did not help. I would recommend a gas cap from the dealer .
The fuel tank over-fill control valve, often referred to as the “roll-over valve” is a common failure on Toyota and Lexus vehicles, they crack and cause an evaporative emissions system leak. Someone unfamiliar with this type of repair or vehicle might overlook a vapor leak on the top of the fuel tank.
I have some shameful coworkers that have many return visits for evap. system leaks after replacing the fuel cap on one visit, the the canister on the next and the leak is still not fixed, those guys aren’t even trying to find the leak (charcoal canisters rarely leak).
We have a smoke machine in the tool room but it isn’t needed for finding most leaks, a combustible gas detector can be used to find the leak, they are not expensive, every Toyota technician should have one in their tool box.
EVAP codes can be the bane of many a technician…especially if they are truthfully trying to fix the condition. The systems are so sensitive and have a lot of places the system could go “open” to the atmosphere… so the computer freaks out and throws a code… Big problem come inspection time. These issues rarely cause any driveability problems however…the code is basically because of the rules for emissions…they are strict and getting stricter.
You need a shop that is willing to find the exact root cause…because many shops like to throw parts at the problem…and this is expensive and usually doesn’t work. This is where the smoke machine comes into play once the shop exhausts all normal and logical troubleshooting…
Also…off the top of my head @abe.clabby_151869… P0420 ( a code my VW loved to throw) is “Catalytic Efficiency below threshold”… This is usually caused by an O2 sensor…and or a leak in the exhaust system…it rarely identifies a cat convertor as being faulty…so suspect that dead dead last.