Coolant warning light on cold days

joelbert · January 23rd, 2013, 16:28

In the last few days it has gotten colder here in NJ. With temps in the mid teens (°F) the coolant warning light flashes after I start the car, and continues to flash for the first trip of the day. After I stop the car and restart it the flashing goes away. Yesterday I did a long trip with it, got to full operating temperature, and the light did not stop until I turned the car off and back on. Today I did a 2 mile trip as the first of the day and same thing, the light did not come on after the first trip of the day even though the car was still pretty cold. I am turning the key, letting the glow plugs glow for about 25-30 seconds and then cranking. It starts in a few seconds, and idles kind of rough at about 1100 rpm for the first 40-60 seconds with a fair bit of white smoke. Then the idle calms down a bit, and I drive off.

According to the owners manual the light indicates that the car is low on coolant (it isn't) or the car is too hot (15°F today when I started it). I checked the coolant reservoir today before I started it, and there were no signs of ice, so I doubt that the coolant mix is way off and freezing to confuse the sensor. A couple weeks ago I used vag-com to check that the coolant and fuel temp sensors were working okay and they seemed fine, but that was starting from about 40°F

Any ideas?

Ski in NC · January 23rd, 2013, 16:36

Coolant level in normal band as viewed min/max stripes on side of bottle?

Gunk can get on bottle level sensor probes. Sometimes these can be cleaned by sticking an old toothbrush in fill hole. Two metallic rods originating from plug on side of bottle.

Art van Law · January 23rd, 2013, 16:43

Yeah, same thing happens w/ my '03. Any time temps are low, say <25°F the chime and light come on upon first start in the morning. I too have checked the coolant reservoir and found the level ok. All I do to cxl the light after 5-10 minutes of running is turn off the ignition and upon restart, the light stays off.

I'm guessing the sensor in the coolant reservoir is malfunctioning, and there's no significant repercussions?

joelbert · January 23rd, 2013, 16:46

Actually a little above the max band. Has been at about that level since I got the car in late August. I haven't done anything that involves the coolant, except bumping the engine coolant sensor a few weeks ago when I reinstalled the tandem pump. I noticed a little coolant drip the next day, wiggled the sensor in it's mount, and the leak went away. I do have a spare sensor I got after that just in case.

Vince Waldon · January 23rd, 2013, 16:46

G12 shrinks quite a bit in the cold... so if your coolant level is ok but low it may trip the sensor when cold and then appear to be fine later.... or a few miles down the road when it's warmed up.

So, worth checking the level with the engine stone cold just to rule this one out.

mcracer516 · January 23rd, 2013, 16:50

My 04 Jetta does this every winter on the first cold day/morning, I add an ounce of water or less and it stops.

joelbert · January 23rd, 2013, 16:59

Mine was a little above the upper band when I checked it this morning before starting it. I may try cleaning the sensor. When I checked the fluid this morning there seems to be a filter inside the opening to fill it reservoir, is it removable? I did not spend lots of time studying it because of the temperature.

Is it okay to ignore this if I have coolant, and am pretty sure that the engine really isn't overheating?

Ski in NC · January 23rd, 2013, 17:41

Yea, if coolant temp is ok on gauge, bottle level ok, and you are getting heat out of vents, I'd say safe to ignore alarm.

sisyphus · January 24th, 2013, 15:33

My wife's 02 does this, too. I ignore it.

fruitcakesa · January 24th, 2013, 16:07

My 02 did this for years and all the attempted fixes were NG .
A new reservoir bottle cured it.