Coolant Temp sensor causing no start?

[Pkr1991]

Hello. I drive a 2004 Audi A4 Quattro 1.8t. Within the past week or so I have noticed every now and than my car having trouble turning on (takes about 6-8 cranks) but sometimes its normal and starts after about 2. Could this be because my coolant temperature sensor is faulty? I've noticed my gauge on my dash went awhile ago but I did not think it would affect anything so serious. Just trying to get to the bottom of this and if anyone has any input i'd really appreciate it. Thanks

[Tongpo]

A faulty CTS can cause starting problems. I experienced it last Fall.

[hightime80]

I've noticed my gauge on my dash went awhile ago but I did not think it would affect anything so serious.

Wow. It's kinda serious. Nothing to warn u when u overheat and bust a HG and warp your head.

[old guy]

The rear CTS is a four pin sensor. It's essentially two sensors in one. It sends a signal to the display for the coolant temperature gauge as well as a signal to the ECM for monitoring the mapped cooling system and also for maintaining the proper fueling requirements. It can most certainly have an affect on starting the engine.

It's kinda like an old school choke plate. When the engine is cold the choke plate closes to richen up the A/F ratio to start the engine. With modern electronics the ECM adjusts the A/F ratio based upon the block temperature as reported by the rear CTS. If it reports incorrectly the ECM may not add the required fuel increase for starting a cold engine.

[Pkr1991]

Thanks for the help everyone. I'll be ordering a new one from the dealer tomorrow. I've watched a DIY and doesn't look so bad so i'll give it a shot.

[Pkr1991]

Another dumb question before I go ahead and swap this out tomorrow. After taking a look earlier and seeing how tight it is to get to the sensor I took apart a J pipe right above the combi valve. Does anyone know what part # this is? Im sorry i do not know the correct name for it as I am still learning. My combi valve was covered in oil due to the pipe above it being cracked and taped up by a previous owner. Just trying to figure out what this pipe is called so i can replace it as I replace the sensor as well. I can get a picture tomorrow if i am making no sense.

[diagnosticator]

That cracked tube could be involved with the hard cold starts from an intake air leak effecting cold air/fuel mixture being to lean for cold starts.

[Pkr1991]

Okay thanks. I'm not sure what its called so i'm going to try and just take a picture and take it to the dealer and see if I can find one around the right size.

[old guy]

Would you be referring to the curved hose connecting the hard pipe to the PRV? (PRV = Pressure regulating valve). That would be the round puck shaped valve.

[old guy]

Let me make it a little easier. Identify your hose from the diagram below and then click on the appropriate number here: Clicky click® to get the part number.

[Pkr1991]

Yes old guy. Thanks for the help everyone!

It was in fact #19 I was speaking of, now I can get everything I need.

[Pkr1991]

So I changed out the sensor today...dam is that a bitch to get that clip on. Anyway...after driving it about 5 minutes everything seemed back to normal but than it started moving down and back up to the middle out of nowhere and than finally went back to C where it was before I changed this out.

Any thoughts on this?

[old guy]

Could be a faulty thermostat. Do you have the capability to monitor measuring blocks in your ECM? VCDS or similar?

[BerNur]

are you auto or manual?

[Pkr1991]

Could be a faulty thermostat. Do you have the capability to monitor measuring blocks in your ECM? VCDS or similar?

That's what I was thinking...also no unfortunetly I do not. I'm hoping it's not my thermostat but I am so confused considering my car does not show many signs of a faulty thermostat as far as my knowledge goes. It was textbook coolant temp sensor to me..the gauge even worked for awhile after I replaced it then kicked off again.

- - - Updated - - -

are you auto or manual?

Auto

[old guy]

So I changed out the sensor today...dam is that a bitch to get that clip on. Anyway...after driving it about 5 minutes everything seemed back to normal but than it started moving down and back up to the middle out of nowhere and than finally went back to C where it was before I changed this out.

Any thoughts on this?

Is your radiator fan coming on immediately when you start your engine? When you have a fault in the cooling system the ECM will run the cooling fan continuously and also apply power to the thermostat heater as a cautionary measure to protect the engine from overheating. It will continue to do this until the code is reset.

You can reset your ECM by simply unplugging the battery for about 15 minutes. Once you hook the battery back up turn the key to the on position and leave it there for two minutes before turning it back off. After that you just start your car normally and any active codes will have reset.

[Pkr1991]

My radiator fan does come on immediately when I start my engine and stays on. (Thanks for that information as that explains a lot.) I'll start by getting rid of the codes and see where I stand. I just find it very weird how the gauge started working again and died out 5 minutes even after I replaced my CTS. I've tried searching for people with this common issue but everyone's case is so different. I'd like to make sure all the connections in the cooling system are intact before I go ahead and replace the thermostat. (I'm not sure if any wires being messed up would cause my gauge not working) What is still concerning me is that the temp gauge did start working slightly and kicked back down to C afterwards after fighting to stay within normal ranges.

Is your radiator fan coming on immediately when you start your engine? When you have a fault in the cooling system the ECM will run the cooling fan continuously and also apply power to the thermostat heater as a cautionary measure to protect the engine from overheating. It will continue to do this until the code is reset.

You can reset your ECM by simply unplugging the battery for about 15 minutes. Once you hook the battery back up turn the key to the on position and leave it there for two minutes before turning it back off. After that you just start your car normally and any active codes will have reset.

[old guy]

Do not replace anything else until after you reset your ECM. Your fans on high indicate an active code for the mapped cooling system. In addition to the fans running on high the ECM is also applying full power to the thermostat heater. This forces the thermostat to open and stay open which mimics a bad thermostat since it prevents the block from reaching full operating temperature.

After the reset you may find that everything falls back into place.

[Pkr1991]

Just finished doing the reset. After I started it up the CEL went back on. Radiator still running full blast. Going to grab codes now.

Do not replace anything else until after you reset your ECM. Your fans on high indicate an active code for the mapped cooling system. In addition to the fans running on high the ECM is also applying full power to the thermostat heater. This forces the thermostat to open and stay open which mimics a bad thermostat since it prevents the block from reaching full operating temperature.

After the reset you may find that everything falls back into place.

[Pkr1991]

Okay so just ran the codes which came up after resetting them from disconnecting the battery. Codes are P0118 which I can't understand why because I just changed the CTS. P0322 which is crankshaft sensor right?

Any idea what my next step should be? I erased the codes and haven't popped back up in about 10 minutes of idling...also my temp gauge is still sitting at C.

Just finished doing the reset. After I started it up the CEL went back on. Radiator still running full blast. Going to grab codes now.

[Pkr1991]

Update :

After checking the codes and one of them being P0118 I knew something wasn't right. I got it home and popped the hood and yes I know this isn't the best way to test it but I pushed down on the sensor and after I noticed the fan stopped and the cars temp gauge started working. So I'm guessing I just didn't put it in correctly. I now need to take it out and try and reinstall it the correct way. It seems to be in pretty well but not enough to catch a temp.

[ozMatt]

I think if the sensor is not fitted correctly you'll be spewing coolant from that point. (I believe the securing clip won't go on properly/will break if the sensor is not inserted all the way). If pushing on the sensor had an effect i would be leaning towards the electrical connection to the sensor. Could be the plug needs a clean or maybe some of the wires have cracked and are shorting out.

[Pkr1991]

Figured out the issue. The clip wouldn't catch on the sensor... No idea why as I tried it on my old one and it catches fine. Overall I ended up having to use a little electric tape to get the sensor to catch on the tab and it's been holding up well.

Thanks for all the help everyone. Old guy you really saved me on this one.

I think if the sensor is not fitted correctly you'll be spewing coolant from that point. (I believe the securing clip won't go on properly/will break if the sensor is not inserted all the way). If pushing on the sensor had an effect i would be leaning towards the electrical connection to the sensor. Could be the plug needs a clean or maybe some of the wires have cracked and are shorting out.

[SHR]

Great thread, thank you. I just picked up an 02 B6 1.8TQA with an intermittent temp reading as well and plan to re-seat the connector and spraying Deoxit on the connection points. I also have a B5 that had a similar issue. The fix was simply spraying Deoxit on the temp sensor connectors, the Deoxit de-oxidized and lubricated the connection so the plug went on with a good solid click whereas it felt like I was forcing it prior to spraying it. Perhaps spray a little Deoxit on the connector if you go in there again.

[Pkr1991]

Thanks! I'm sure the electric tape helping hold the clip on won't hold forever so I will give that a try if I notice an issue. By the way if anyone else is wondering all I did was stick electric tape inside the clip to make it stick. Been running great sense.

Great thread, thank you. I just picked up an 02 B6 1.8TQA with an intermittent temp reading as well and plan to re-seat the connector and spraying Deoxit on the connection points. I also have a B5 that had a similar issue. The fix was simply spraying Deoxit on the temp sensor connectors, the Deoxit de-oxidized and lubricated the connection so the plug went on with a good solid click whereas it felt like I was forcing it prior to spraying it. Perhaps spray a little Deoxit on the connector if you go in there again.

[customa4]

Curious, did you use an OEM temp sensor or aftermarket?

[Pkr1991]

I went OEM. I did a ton of research and many people complained the after market sensors blowing out after short periods of time. Also, as anyone would know after replacing the sensor....it's not too fun and a real knuckle banger. I also picked up 2 clips just incase I dropped one and I ended up dropping one so I would suggest getting two.

Curious, did you use an OEM temp sensor or aftermarket?