EPC light only when raining AND slippery

Hi. I have a 2006 VW Rabbit with 106k miles. It is an automatic. Strange problem: if it is raining AND the traction control symbol comes on (I slip), then the EPC light comes on and the car is slow to shift and has poor acceleration. This does not happen if it is not raining and it does not happen just when it’s raining. One mechanic recommended cleaning the throttle body, but since the car runs great all other times, this didn’t make sense to me. Any ideas?

Question: Has it always behaved this way or did it just start doing this? It is normal for a traction control system to limit engine power to the wheels when they begin to spin, resulting in perceived poor acceleration. Depending on the programming, it may also be designed to hold a lower gear longer than normal if it detects wheel spin.

If this is not the normal behavior for this system, I’d probably start by checking the wheel sensors that measure the rotation rate of the wheels. If one or more is giving erroneous inputs to the computer, it will confuse ABS, traction control, and stability control systems. That it is occurring in damp weather may mean that water is penetrating into the sensor’s coil or into some degraded wiring. The computer may be able to ignore or ‘filter out’ some spurious signals from a flaky sensor except in conditions where the system is frequently engaging–like when it’s slippery out.