The dash display went on and off intermittently, car still ran without trouble mechanically. Still as this is a car with an all electronic display not knowing your revs or speed is a bit risky, so sensibly she pulled over, stopped the car and called me.
Skip to the end for wrap up, symptoms, result etc.
Unfortunately following this the car would not start, however the battery appeared in fine condition, lights worked and turning the key in the ignition turned on the radio, but still no display, with full control over aircon, etc, but car wouldnt even try to turn over. Only warning sign was the Coolant light on in white. She mentioned she thought the car smelled funny beforehand, so suspected overheat, however upon checking the coolant was not low. Further reading later proved that the coolant light is on in white when the car is cold (it was about -1 out) and goes off when the cars warmed up. This is however the only thing we had to go on so first port of call was checking that the car doesnt 'lock you out' if it over heats. This is not the case (also, car hadnt overheated) - Smell hasn't come back since so may have been car infront or similar - we recently changed the exhaust so i told her to be wary of odd smells etc, just incase the fix wasn't perfect.
Sometime later with the key in the ignition she slammed the passenger door and the display jolted to life - turned the key to turn on the engine and it started first time, no problem - we drove to my fathers house as he has off street parking. Once there, it once again wouldnt start - no display.
This was clearly however an electrical issue. The car ran fine, but no display meant a power problem which was stopping the car starting. First check was to run home to get the spare key incase it was an immobiliser issue. Same result with the spare key.
Gave up for the evening and spent the next day (in daylight) checking fuses, switching out relays, attempting jumpstarts etc and trying to find the cause.
Eventually noticed that upon trying to turn the key to start the car caused a very faint and intermittent clicking sound for the area of the display itself. People will tell you when trying to start a car if you a hear a click click click this is a solenoid failure and the car won't start. This is likely the case if the car is constantly clicking loudly from under the bonnet while trying to start, but this doesnt fit with the lack of display (and working radio etc) nor was it coming from the right place.
Eventually with nothing else to go on i resolved to take the display apart and find the source of the noise. I've done my fair share of soldering so i was hoping for an obvious blown capacitor or something. Taking the dash apart was surprisingly easy (i did it all with a small penknife) - the cover comes off by pulling towards you and up, revealing the display section underneath which is held on the car with 4 screws. Taking this out i noticed no obvious changes with unplugging and plugging back in connectors (although unplugging the display caused the ABS warning light to come on....)
A few bits of wire jiggling incase something was loose proved fruitless - word to the wise, i have no idea how this works, but the airbags are in there somewhere and can deploy with such force they can kill you so dont go messing around behind the dash too much.
Anyway, finally got down to the circuit board itself (only a few clips and 2 screws involved) and it appeared the noise was coming from the speaker (small round plastic component with a hole in the top). Not quite the obvious short i was looking for.
However years of messing about with guitar speakers and pedals got my brain going and a quick google proved my theory that a clicking speaker is an indication of a grounding problem..... plugged all back in solidly and tracing the board back proved no issue, so it was back to outside the car.
Connection to the battery negative was fine and thats connection to the car chassis was solid. However another cable goes down from the battery negative in to the lower part of the engine bay. A quick yank on this proved it was loose - slid back in to slot and i heard the relays in the fusebox click. Checking the car again the display came on and it started first time. Success!
Result: basic grounding problem from the battery. Extremely simple - check the battery connections and the connection from there in to the engine. I can only assume that because the rest of the car is also grounded to the chassis this was enough to power up the lights/radio and all those things, but the electrics of the ECU works on a separate ring/require more voltage that it wasn't getting. Being that these cars are, despite their simiplicity in terms of mechanics, entirely reliant on the computer to start the engine, a dodgy connection (presumably assisted slightly by earlier door-slamming) proved enough to stop the computer starting which meant the car wouldnt go anyway. Thankfully a damn simple problem, damn simple to fix, but took a while to find.
One small problem - putting the display back together properly proved that it worked fine, showed rev's fuel gauge etc, but Speedo was not registering the speed. This in turn caused the engine warning light to turn on.
removing the postive terminal of the battery for 30seconds reset the light, and some combination of checking the connectors to the display circuit board and pressing the kph/mph buttons and odo button brought it back to life. Works fine now and engine warning light hasn't come back on.
UPDATE: Turns out this wasn't a push-fit connector, a completely broken connection which some one else previously tried to fix with electrical tape - presumably pushing it back together over the sleeve of tape touched the contacts for long enough to start the car. A couple of days later it broke completely. Thankfully all it is is a ring connector from the battery to a bolt on the outer side of part of the engine. I purchased the replacement cable from Halfords (UK) for £5.00.
Fitting required removal of the battery and 2 nearest hoses from the air filter (one turns and pulls out, the other needed the clamp loosened...) then just unbolt the broken connector, connect back to the negative battery terminal and hopefully we have a permanent fix.
IF YOU GET STUCK - (as we did on Thursday) you can fix this in the meantime by using a jumpcable to connect the negative terminal of the battery to any metal part of the engine block (although clipping to the broken connector worked for us to drive 20 miles home).
You will need to leave this connected while you drive - without it the alternator will not charge the battery, it will slowly rundown and once it hits bottom the car may conk out (this may be specific to Automatics though - usually you can drive cars with dead batteries if you get a jump start, but not in our case)
Right, story done, time for the:
Symptoms: Yaris dash display went out while driving. Once stopped, would not re-start. Turning the key turned on the radio etc, lights and heater worked, but the display showed no life (note, engine lights to the side of the display are not part of the display itself).Eventually noticed faint clicking coming from the display itself.
Checks: Fuses all Ok, some relays can be swapped but this did not help. Battery fine (proved by lights etc working). Trying spare key didn't yield any different result. Took dashboard apart to discover faint clicking noise coming from speaker on display's circuit board.
Result: Speaker noise suggested grounding problem - check connection to the negative terminal on the battery under the bonnet, then from there to the chassis (cable 1) and most importantly - second cable from the negative going in to the engine bay - A loose connection here was my issue -
Couldnt find this covered anywhere else on the web, so hopefully this will help someone!