Modus glow plug light and engine smoke

I also drive a Renault Modus (though not the same year), and I had similar symptoms. The glow plug light staying on and the engine light usually points to issues within the preheating system. It could indeed be a faulty connection, but it's difficult to say for sure without further testing. Did the diagnostic test give you any error codes?

The diagnostic test indicated error code DF007 - Glow Plug Circuit Malfunction. Does this confirm your suspicion of a cable or plug issue?

DF007 definitely points to the glow plug circuit. While a faulty cable or plug is a strong possibility, and a common cause, it could also be the glow plugs themselves or even the glow control unit failing internally. Given the potential for multiple issues, I'd advise having a workshop take a closer look. You don't want to risk engine misfires or further damage, especially with the smoking issue.

Understood. I appreciate the advice. I took it to the workshop as suggested. Turns out it was indeed a corroded connector on the glow control unit, costing 95€ to repair. Thanks again!