Is P0405 putting this vehicle in Limp Mode?

A continuation of the saga with my neighbor’s 2010 Peugeot Partner II 1.6L HDI Turbo Diesel.

Basically, one of the various problems that I diagnosed with the vehicle was a defective EGR, which I’m absolutely sure of. It had a bunch of EGR codes, but P0405 was not one of them. I ordered a new EGR and installed it, and immediately got a bunch of EGR codes when I plugged it in. I unfortunately didn’t write them down. I cleared the codes again and got the P0405 EGR Circuit Low EGR Repeat Raw Signal. It’s the only code in the system and shows permanent.

As noted in the linked question above, there are some other suspected issues such as a blown head gasket or dirty injectors. After installing and hooking up the new EGR the vehicle would barely start and once started and warmed up to operating temp started refusing to go above 950 RPMs. It’s also belching black smoke like there’s no tomorrow. Prior to changing the EGR it was at least drivible, even if it was a bit down on power and had a rough idle.

Leaving aside the issue of why this new code came up, can I assume that it’s putting the ECU into limp mode and that’s the cause of these new symptoms? The above link indicates that this code may put the vehicle into limp mode, but is that a required behavior by OBDII or is it manufacturer specific? Also, given the symptoms does limp mode as a cause make sense, or is it just conincidence and maybe the suspected underlying issues just suddenly got worse?

I left the vehicle overnight, disconnected the EGR from the ECU, tried clearing the code but it won’t go away.