Fault codes 01314 AND 00526

[1611AZTEC]

TDI 2003 GOLF ASZ PD130. 157k miles.
Car owned for 4 years with usual tale of Golf faults and had to changed clutch in December after a failed slave cylinder.

Had a problem starting after a short journey but started on third third try but traction control light stayed illuminated though the car took me home a few miles no problem.
After restarting a few times without a problem at home the TC light went off.

Pulled these two codes, 01314 pointing to loss of power to ECU and 00526 brake light switch both intermittent..
Brake lights check out ok and working and have changed over under dash relay 109 that supplies the power to ECU.

After a change of relay I expected to start right up but for the second time did exactly the same and went third or fourth try and have since tried numerous times with it starting instantly on the button each time.
I suspect another problem other than the relay. Wiring,earths , ignition switch etc are on the list to check next

Can anyone point me towards the most likely or most common cause other than relay 109?
I read this is a pretty common occurrence and maybe somebody can pass on their experience that may save me a long hunt.
Intermittent problems are the worst!

Thanks in advance. PF

[Crasher]

01314 - Engine Control Module: No Communications

00526 - Brake Light Switch (F): Circuit Malfunction

The brake light switch also incorporates a brake pedal switch, the action of light switch sends a signal to the lights and the ECU, the pedal switch just sends a signal to the ECU but these signals must match, failure of this switch was rife when the Golf 4 was current, we often fitted four a day! There are two designs of replacement switch, one fitted pedal down the other pedal up, when you buy one you need to ask which type it is but that is likely to illicit "duunno" from the parts person, genuine VW switches are pedal up. The 109 relay was a good idea, if the fault comes back, yes I would suggest an ignition switch but they are not a common failure on the Golf 4 unlike the Golf 5.

[1611AZTEC]

Thanks Crasher. Yes understand the brake switch not really sure why the ECU needs to see a signal fro the brake switch but there is sure to be good reason maybe to shut down engine on braking?.
The main problem I really need to solve though is the 01314 as its preventing the car from starting first time as its a really good starter. Usually starts on first crank but a few times now I get the non start for two or three attempts and then it finds a path to start and runs fine though havent taken it too far because Im not sure It will develop into a total non starter and get stuck somewhere. First turn of ignition I have all dash warning lights including glow plug light momentarily I wait for all lights extinguish and crank the engine. The first time it happened I was left with the traction control light on for the journey home but when I switched off and back on again engine fired up and traction control light went out. I would imagine the momentary loss of communication or power to the abs module on cranking may have caused the light to illuminate. The fault code was in the was logged in the ABS diagnostiC setion of my laptop software not in the engine section which was clear. Have changed the 109 relay and on first start up after changing it didn't start but did after a few tries so more or less certain the problem lies elsewhere.
Hate electrical problems as I'm a old school career mechanic. I have the ECU pin out data and and the chassis engine earth locations along with each wire colour code positives and earths (brown red tracer) wires going to the ECU with terminal 18 (yellow) the one that should have positive battery voltage with ignition on. Being an intermitent fault though I can only check and clean all of the connections for now and hoping it may be an earthing fault to the ECU and may have been disturbed when I changed the clutch in December. I did also see through the plastic scuttle tray where the ECU hides another multi earth point which I would imagine is to supply the ECU and comes through though the loom trunking from under the battery tray where the main grounds are fixed. Are they known to corrode and fail?
Its an odd one and Ill have to work my way through it or give the car away! Any more experience you may know of of similar problems would be greatly appreciated. Thanks again. PF