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Deposits accumulating in the throttle bore and around the plate's sealing edge, gradually changing where the plate sits when closed. The build-up matters at idle more than anywhere else, because the module controls idle with very small plate movements and the carbon occupies exactly that range.

It is fed by the crankcase ventilation and recirculation systems, which route oil mist and exhaust into the intake immediately upstream, so accumulation is normal rather than a defect. The result is a creeping idle fault — hunting, stalling on overrun, or an idle that sits high — with a throttle body that is mechanically sound.

Cleaning has two conditions that are routinely ignored. Many electronic throttle bodies carry a coating that solvent or abrasion will remove, making the surface rougher and the deposits return faster, so use only what the manufacturer permits. And a relearn afterwards is not optional: the module has adapted to the old closed position over years, and removing the carbon moves it. Skipping the relearn is why a correctly cleaned throttle body sets new codes immediately.