Reading real CPU temperature on Intel and Apple Silicon Macs

How the SMC reports temperature

The SMC exposes sensors as four-character keys, each with a type that tells you how the bytes are encoded. Get the wrong decoder and the number is garbage, so the format matters as much as the key.

On Intel Macs, temperatures come back in the sp78 fixed-point format: a signed value with eight integer bits and eight fractional bits. On Apple Silicon, the same readings come back as IEEE flt floating point. Fan speed has its own encoding too, usually fpe2 or flt depending on the machine.

So even before you ask which sensor to read, you have to know which decoder to apply for that Mac. LidRun reads the keys directly and decodes sp78 or flt as appropriate, rather than guessing.

Why there is no single 'CPU temp' key

You might expect one tidy key called CPU temperature. There isn't. Different Mac models expose different sensors, and the naming differs by architecture.

Intel Macs use the TC0x family of keys for CPU core sensors. Apple Silicon is more fragmented: it exposes many per-cluster keys in the Tp__ and Tg__ families, covering performance cores, efficiency cores, and the GPU, because the chip simply has more distinct thermal zones to report.

Because no single key is present on every Mac, LidRun tries a list of candidate keys and takes the hottest valid reading. That sounds blunt, but it is the honest approach: the hottest live sensor is the one that matters for a thermal limit, and probing a list survives the model-to-model differences far better than betting on one name.

On top of the numeric reads, LidRun also looks at ProcessInfo.thermalState, the coarse signal macOS itself publishes. It only has a few levels and is not a temperature, but it reflects how the system is feeling about heat, which is a useful sanity check next to a raw sensor number.

Why two apps show different numbers

If one tool says 68 and another says 74 on the same Mac at the same moment, neither is necessarily wrong. They are almost certainly reading different sensors, or picking the representative number a different way.

One app might average a set of core sensors; another might report a single package sensor; a third might do what LidRun does and surface the hottest valid key it found. Those are three reasonable choices that produce three different numbers from the same hardware.

The lesson is to treat any single CPU temperature as a reading from a chosen sensor, not as one true value. What matters for safety is the trend and the peak, not whether your number matches a friend's tool to the degree.

Reading temps is not the same as controlling fans

Reading is the easy direction. Writing back, telling the fans to spin at a chosen RPM, is where the hardware draws a line. On Apple Silicon, fan RPM write-control is largely restricted by the firmware and SIP, so manual fan curves on M-series Macs are mostly off the table. This is a real platform limit, not a missing feature.

LidRun is honest about that. It reads temperatures and fan RPM on both architectures and offers cooling profiles within what the hardware allows, but it does not promise manual fan curves on Apple Silicon, because it cannot deliver them. Tools like Macs Fan Control face the same wall on M-series for the same reasons.

Practically, this means temperature on Apple Silicon is mostly something you respond to rather than override. Knowing the real number lets you set a sensible thermal threshold and trust the safety governor to back off when it is crossed, which is the part you can actually control.