The temperature gauge dropped to zero within few seconds, stayed there for few seconds and then came up to normal running temperature within few seconds. All very noticable; that's how I noticed it when I was not actually looking at the gauge! I am assuming it is the water temperature that's been measured. Now, I can't imagine the water temperature suddenly cooling significantly then heating up again in few seconds, can it? Is it possible that a failing oxygen sensor can cause some malfunction that temporarily fools the temperature sensor... Or maybe I am talking rubbish.
I don't have much experience in this area. The last time I saw thermostat failing, it was my son's Astra-G, z16se engine. The symptoms were classical. The engine took ages (15min or so) to warm up to 70C(!) when stationary and the needle dropped down to the limiter as soon as the car started moving. This took just under 2 minutes. The thermostat was stuck-open, which is a typical failure.
Now about your daughter's car. I've checked the pin diagrams of the ECU. Both sensors are connected independently to the dedicated ECU pins. It is unlikely to have a fault that affects both, unless it is an ECU fault or power. The O2 sensor can only cause the ECU to adjust the injection charge or timing. It will have an effect on the temperature, but the thermostat is supposed to handle it. So, I don't think that the O2 sensor can be responsible for this fault.
On the other hand, the temperature sensor fault may cause increased temperatures of exhaust, which may affect the O2 sensor and the Cat, in theory. Again, I am not an automotive professional, so it is just my limited understanding of things.
Guys just to let you know that the car will be in at the dealers next week. There are a few things that could be wrong with the car, thermostat, oxy sensor or a new one a coolant heater. Anyways I'll keep you all posted on what they find.
So Good We have two. Well that's what I Thought at First:
Diggin up an oldie but it may be of use to someone.
I also had this problem with my 1.4. It was coupled with a failure to start when the engine was warm (i.e., not hot but left for 10-15 mins). My garage could diagnose neither problem despite repeated attempts (apparently). However, one morning, I was able to drive my car to the garage and show them it running with the engine warm and the guage showing zero. It even gradually rose again as the service manager was looking at it. "Right, we'll change the coolant temp sensor." This they did and it solved the problem. I was also told that they recreated the start problem (funny how they were now able to manage this ) before changing the sensor and found it to be showing a coolant temp of -20C! Anyways, you wouldn't have thought it would have taken a genius to work out that there may have been a problem with the sensor but if they don't see the problem 1st hand, they won't investigate anything.
Sorry to dig this back up from the dead, but I have the exact same situation as 72uoba had.
Sometimes it takes upto 6-8 attempts for it to start when warm, and to keep it from stalling, the only way is to keep the revs up at a high rate.
Will this be covered under the Reagroup extended warranty?? If anyone knows, I would be grateful if you could let me know, as so far the warranty has failed to pay for two window regulators and a door seal which I could have done without paying for at the time, it seems nothing is covered under the warranty and I have wasted hundreds on a worthless piece of information on a computer.
Megane II - 1.4 16v Dynamique 5dr - 2004 (UK
Scenic II - 1.4 16V Dynamique - 2004 (UK)
Scenic II - 1.5 dCi 86bhp Latitude - 2008 (France)
I had a similar problem, engine temp gauge dropping off, I had the UCH replaced and I haven't seen it since and that was in May '07. (The garage had previously checked to see if it was the engine temperature sensor but it tested ok). Regards,BG
I had this problem before - temperature guage dropping off and then returning to normal at random. Garage replaced coolant temperature sensor once and it seemed to solve the problem. However, almost a year later and the same thing started to happen again! Went into dealers twice but no faults showed. So, decided to have a look myself. Removed electrical connector and found green corrosion. Brushed it off and sprayed WD40 into both sensor and plug and re-attached. All has been fine for a month now. Worth a look if you're still experiencing similar problems.
I am having a bit of an issue with my temp gauge as well.
It takes a long time to start rising about 5 minutes and no longer sits at the normal running temp (3rd bar along) it continually sits between the second and third bar and the gauge.
The only time this will get up to where it usually sat is if I am driving over 2 hours or have been driving in the hills. Do you think it could be as easy as a new coolant thermostat?
And advice appreciated
-Aussie Renault addict.
(EDIT) I should add that I have no issues with starting or sluggish behaviour from the engine.
had proplem starting when hot. started fine then stalled ,even when left for 10 mins . you could catch it with throttle before it salled . solution -cleaned throttle body of gunk film built up.just that.solved it thanks for help everyone
Or maybe I am talking rubbish. 
) before changing the sensor and found it to be showing a coolant temp of -20C! Anyways, you wouldn't have thought it would have taken a genius to work out that there may have been a problem with the sensor but if they don't see the problem 1st hand, they won't investigate anything. 
