P2024: Evaporative Emissions (EVAP) Fuel Vapor Temperature Sensor Circuit

It is easy to file this under emissions and assume the consequence is dirtier exhaust. It is not. The real consequence is a test that no longer runs.

The evaporative system's job is to trap fuel vapour in a charcoal canister instead of letting it escape, and the module's job is to confirm periodically that the system is sealed. It does that by sealing the tank and watching the pressure — either pulling a vacuum and checking it holds, or letting natural vapour pressure build and checking it does not bleed away. Either way the entire test is a pressure measurement on a closed volume.

And pressure in a closed volume full of fuel vapour is dominated by temperature. Warm the tank a few degrees and pressure rises on its own. Cool it — park in shade, drive into cold air, refuel with cold fuel — and pressure falls with no leak involved at all. Without knowing the vapour temperature, the module cannot separate a genuine leak from ordinary thermal behaviour, and a leak test that cannot make that distinction is worthless. So it declines to run it.

That is why P2024 usually arrives with no symptom other than the warning lamp. The engine does not care about vapour temperature. Fuel delivery, ignition and idle are unaffected. What you will discover, typically at inspection time, is that the evaporative monitor sits at not-ready and will not clear no matter how much you drive, because the module is not attempting the test. In many jurisdictions that alone fails the inspection, which makes this a low-severity code with a hard deadline attached.

The sensor itself is a thermistor rather than the divider-style sensor used for position measurement. It has no moving parts and no reference supply of its own — the module applies a voltage through a fixed internal resistor and reads the point between that resistor and the sensor. Resistance falls as temperature rises on the usual negative-coefficient design, so the module reads temperature as a voltage. Two failure signatures follow directly from that arrangement: an open circuit anywhere leaves the module reading its own pull-up voltage, which it interprets as an implausibly cold temperature, while a short to ground collapses the reading and it interprets an implausibly hot one. Both extremes are out of range and both set a circuit code, so the direction of the reading tells you which of the two conditions to look for before you have measured anything.

There is one test worth doing before any of that, because it is free and it settles the question of whether the sensor is honest. Leave the vehicle overnight, then read the vapour temperature first thing alongside ambient air temperature. A tank that has sat unrun for eight hours has equalised with its surroundings, so the two numbers should be close. A reading that is wildly apart from ambient after a cold soak condemns the sensor without dropping the tank. A reading that agrees with ambient points at an intermittent connection instead — which matters, because on many vehicles this sensor sits in or on the tank, and knowing before you start whether the tank has to come down is the difference between a small job and a large one.