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I am getting a P0455 (Large EVAP Leak Detected) code on my 2008 Chevy Cobalt LT. As always, one of the first things to check/replace is the gas cap, which I’ve done. I cleared the codes and began driving, going through the GM recommended drive cycle and a few cold starts to get all of the sensors back into a ready-status. However, it’s still showing one or two (one of which is the EVAP) as being in a “Not Ready” status.
I recall a few years ago when I had this issue that a mechanic told me if the gas tank is filled above or below a certain level (something like above 3/4 or below 1/4 of a tank) that the sensor won’t read/report anything until a certain level is reached. Is this true? If so, which of the two theories is correct?
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There are a number of parameters that need to be in range for EVAP monitors to run.
Found this on a chevy forum:
Conditions needed to run vacuum EVAP tests
Fuel heated begins to produce pressure. A rise in temperature occurs as the fuel pump gives off heat. Ambient temperature is also a concern. The GM and Ford systems do not use a fuel temperature sensor. Instead, they check intake air temperature (IAT) and compare it with engine coolant temperature (ECT) reading. If they are near each other, the vehicle has not been running. Engineers assume the vehicle has unheated fuel, when meeting these conditions.
Before the tests run, the PCM also checks the vehicle speed and engine load. The tests require the engine to be running, at an idle, with near a zero load. This means there will be a steady vacuum, and fuel will not be sloshing in the tank. Ambient temperature is also considered and must be above 40 degrees Fahrenheit. A high limit, of 100 degrees Fahrenheit, is set for most models. A few vehicles have the upper limit at 86 degrees Fahrenheit. If the temperature is outside these ranges, the tests will not execute.
Other factors include barometric pressure and fuel level. The fuel level must be between 15% (1/4 tank) and 85% (3/4 tank) and steady. If the fuel level is changing, as with sloshing, the tests will not run. The PCM must also be clear of diagnostic trouble codes, to enable the EVAP testing. Because of these specific requirements, the vehicle may not test the EVAP system on every drive-cycle. This can cause the check engine light to turn off temporarily, although a code is still present in history.
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