Peugeot 508 2013 508sw, drivers side heater only blowing cold air

No idea why it would only affect one side unless there was a fault with the heater matrix config , check this

" I had this issue and it was resolved with a heater core replacement. The test of checking the temperature in the core pipes won't work - the issue is that one half of the core (always the right side) gets clogged and no coolant flows through it. However because coolant still flows through the rest of the core, the pipes still get hot on both sides.

If you look at a cutaway diagram of the cooling system it makes more sense - each zone (left and right) are channelled separately through the heater core, so when the right side clogs the air doesn't heat up on the right side. It's clearly a design flaw because it's ALWAYS the right side that clogs up. If I recall correctly the inlet and outlet are both on the left side.

@Bruce.H your mechanic friend would be right in the case of most cars, but the particular flaw in this car means the problem is far more likely to be the heater core. In most cars you wouldn't get a case of a blocked heater core affecting only one side, but the particular configuration of this car means that's exactly what happens. Thankfully the matrix is a much easier replacement in this car vs many other cars (which often require dashboard removal).

It's common in all cars that have this heater core configuration, for example Citroen C4, where you can find more info."