Low power and max 3000 RPM. Error codes U1218,P0691 andP06933

My car had a change of engine some time ago. It has never produced the same power as the previous engine.
There was a problem recently when the radiator cooling fan remained on all he time when driving and for some time after stopping. The engine wasnt overheated as it came on with a cold engine start.
I replaced the two speed regulator item 1529 . The old unit had a broken spiral resistor, so it needed changing.
Despite this I keep getting the following error codes. U1218. P0691, P0693.
The fan motor itself runs if I put 12 V directly to the terminal 1 ,so it isnt burnt out. I have tried connecting a brand new fan instead of the old fan becuse the citroen service dept suggested this could be the problem. The fault remains after the new fan motor is connected. I only tested it with the car parked and engine on for some minutes. I couldnt drive the car with it connected because it was sitting on top of the engine due to short connector length.
The error codes keep returning after deleting them.
I was wondering too if someone could tell me what this black plastic box contains. It is situated fronts LHS. Note that this is not a UK model so it could possibly be situated differently .
Thanks for any suggestions.

If you have the wire wound resistor type fan speed controller then the speed is selected by a relay or relays and the relay is controlled by the engine ECU.
Unfortunately there is a number of ways this is done.
Some of these arrangements have the relay built into the resistor assembly and some have the relay independent of the assembly somewhere else in the engine bay.

If the relay is part of the resistor assembly then you might have a wiring problem.
If the relay is elsewhere, it might be the relay.

Thanks for your reply. I have discovered that the photo is misleading. These two relays are not used on my C3. They are indeed integrated into one unit with the spiral resistor.
I replaced this unit a few weeks ago and still have the problem as mentioned.

This sounds like a wiring problem.

The system (cooling fans) fail safe is the engine ECU needs to hold the fans off, when not required. Then turn the signal off when the fan is required. If there is no signal from the engine ECU then the fans will run. So if there is some failure between (or with) the engine ECU then the fan comes on as a fail safe.

My first port of call would be the connectors at the engine ECU. Disconnect them and look for corrosion. Failing that, follow the wiring loom from the fan assembly to the engine ECU. Look for damage to the wiring loom.

I take it the service department did not even look at your car? That you just walked in? Yes there are a few versions of the C3 with integrated fan and electronic speed controllers that do fail and run all the time and you would need to replace the whole fan assembly. But they should know that not all versions have that kind of arrangement and a lot have the relay and resistor type speed controllers. Just telling you to change the fan on spec was a bit foolish of them. If they had asked you one question (as I did) or looked at the fan shroud, they would have known it was not the fan or resistors, because the fan was running. Could still have been a relay though (but you have eliminated that).