Hi everyone,

I’m having an issue with my 1999 Mercedes-Benz CLK 320 (W208) and I’m hoping someone can point me in the right direction.

The problem started when the check engine light came on, and I noticed that the secondary air pump would randomly activate while driving (not just on cold start). When I press the gas pedal, the pump immediately turns off.

Here’s what I’ve done so far:

• Replaced the secondary air pump → problem persisted
• Replaced the K40 relay module → problem completely disappeared at first

However, after a few weeks:

• The air pump started turning on randomly again during driving
• It still turns off when I press the accelerator
• Now the check engine light is back on

I checked all fuses and relays in the K40 module, and everything appears to be functioning properly.

At this point, I’m suspecting something may be damaging the K40 relay over time or causing it to behave incorrectly.

Has anyone experienced something similar?

Any guidance on what to test next would be really appreciated.

Thanks in advance!