Multiple error codes

Hi , yesterday got the injector light constantly lit and reading the codes found this: 89-Baro pressure sensor, 07-MAF, 22-Canister valve, 50-Boost Control, 04-02-Idle Control, 16-Timing variator:

Cleaned them and after starting the engine got the same. Now, looking over the wiring diagrams, (my) logic says all these share common positive supply coming from the injectors control relay which should be 12v, and I only measure 6v on the corresponding pins. And few questions arise, because of my 20vt being mid-99 and the relays don't correspond 100% with the diagrams. Am I right assuming the injector control relay is the red one and the fuel pump relay - the black one, just behind the ECU and the glove box (LHD)? Also, there is junction in the diagrams between all these sensors/valves/actuators positive supply and the relay switched contact, but where actually is it? Somewhere in the loom? I think the problem should be there because the relay outputs 12v (if it's the correct relay I measure). And then there are other three relays sitting top right in the engine bay, one red and two blacks, someone knows what they are for?
Cheers👍🏻

The fuel pump relay is the one that only activates for a few seconds to prime the pump when you switch on the ignition, the main injection relay should be on constantly with the ignition. Or trace back from the fuel pump fuse which should be next to those 2 relays.

The multi-junctions in the wiring diagram have the bare wires ultrasonically welded together and will be taped up in the loom somewhere.

Sounds like you may have a bad ground. If you have a set of jumper cables try connecting from the negative terminal on battery direct to the engine or chassis and see if that helps.

Also check ground points for corrosion or loose connections, 6 ground points in engine bay:

1. Front left, on chassis leg near alternator
2. Front right, on chassis leg in front of battery (opposite to #1)
3. Front right of gearbox (cable continues from #2)
4. Right side under coil pack cover
5. Back right of engine bay near strut top mount next to ABS diagnostic socket
6. Rear of engine near bottom of large triangular support bracket (note, bracket lower bolt must also be present for good earth connection)

If that doesn't help I'd disconnect all the devices giving an error, clear the error codes and then re-connect them one at a time with switching on ignition + rechecking for error codes.

Thank you for the detailed answer, Graham. Have checked the grounds - all good, the relays are confirmed being the ones I suggested, the problem is still present, but, at least, now I think the junction has to be somewhere between the relays/ecu and the point where the loom enters the engine bay. There is 4-pole connector attached right there to the main loom which gives the same reading of 5.80-6V, not sure what it is for though and there is no continuity between this and the rest of error-givng devices. Have to find some more free time to investigate that further.

Now solved. There's definitely error or other version of wiring diagrams for the latest production cars, as that finally came out from a FUSE. The 30A fuse attached to the injector relay actually goes after, and not before the relay as per the diagrams, which is not much of trouble if it just blew, but no, it was perfect to the eye, and so was to the multimeter, but not in every position. And that just before got mentally prepared to open the main loom. So new fuse and some soldering of not very tight looking crimping of one of the terminals saved the day.