Hello!
Let me guess - it's an AJM, right?
Since the error doesn't occur even after driving 30km sometimes, I actually think it's not the PD unit.
I wouldn't guarantee that, because there could also be a problem with the connection terminals of the solenoid valve.
Does anyone know how I can check the cables?
Okay, take off the valve cover, and you should see more; the hoses should be connected to the PD elements.
Or, which pin on the ECU corresponds to cylinder 1?
Okay, I'm a bit unsure about this; possibly remove the control unit and look for four identical power amplifier stages (usually large semiconductor components on heat sinks or housings/circuit boards). Then, trace the circuit paths and, for safety, check them with an ohmmeter.
Best regards,