Help on P0128 code - Thermostat Fault

[V7o7]

Unsure what to do next with my 2006 New Beetle TDI - 1.9 Diesel. About a month ago code P0128 was read on my check engine light. I replaced the thermostat ( not fun) and the Engine Coolant Temp Sensor. Still coding P0128 . I thought maybe the plug was bad on the Engine Coolant Sensor so I replaced that. Then I replaced the engine coolant reservoir figured it needed because of age. I tested the water pump by removing the return hose to the coolant reservoir and put the hose end into a jar. Started the engine revved to 2000 rpm kept it there and had return of collant into jar - so I am thinking the water pump is ok. I am at loss at what to do next. Maybe I put the new thermostat in wrong ? Is there a sensor in the housing of the thermostat that can go bad? I have done thermostat replacements before. Any ideas ?

 

[BobnOH]

Some codes must be cleared, some will go away after several runs.

 

[MATPOC]

Does temp gauge read 190 when you drive? I had bad t-stat, was open just a tiny bit and needle never got near the middle of the dial

 

[DaveyT]

I just had the same code pop up which says the "coolant temperature is below the thermostat regulating temperature".
My gauge indicates slightly lower than straight up 190. Does this cause any problems if the operating temp is slightly below 190?

 

[MATPOC]

You could have bad switch or wiring, plug in OBD2 and check what temp it's reading, it's aa different sensor from the one that goes to your dash.
One way to test t-stat is to let car idle till fans come on, with TDI might take a while, note the temp, then go for a ride, if temperature drops dramatically you need a thermostat

 

[DaveyT]

You could have bad switch or wiring, plug in OBD2 and check what temp it's reading, it's aa different sensor from the one that goes to your dash.
One way to test t-stat is to let car idle till fans come on, with TDI might take a while, note the temp, then go for a ride, if temperature drops dramatically you need a thermostat

Thank you, I will try that test.
Is there any potential harm to the engine if its driven with the temperature less than 190?

 

[MATPOC]

It may not operate at it's full potential, with gas cars it will run rich and flood, foul plugs, damage O2 sensors, but with diesel none of those are an issue, just bad heat and maybe not full power

 

[BobnOH]

This era thermostats seem to fail this way, they just get lazy and slow. No harm if it does not reach temperature, but you'll want to get it remedied. Get a genuine thermostat and the plastic neck piece (can't remember what it's called).

 

[MATPOC]

My thermostat failed when rubber seal fell off the plunger and got stuck in it sideways. It's recommend to replace t-stat and water pump with every timing belt.
Car ran OK, but I had no heat

 

[DaveyT]

Thank you for the input folks. With winter coming, I won't like driving without heat. Better get it done I guess.

 

[MATPOC]

Get a plastic neck too, screws just in case, ID parts has a kit.
T-stat swap takes 10 minutes, draining and filling/bleeding coolant takes much longer

 

[DaveyT]

Is this the kit you are referring to?

Thermostat Replacement Kit (A4 ALH)

IDParts has put together a complete service kit for a thermostat replacement on 1999-2003 Mk. IV TDIs. Often times, the thermostat housing breaks upon removal of the thermostat, or a bolt gets lost...

www.idparts.com

If so, with this kit, I won't have to drain and refill the coolant?

 

[MATPOC]

Is this the kit you are referring to?

Thermostat Replacement Kit (A4 ALH)

IDParts has put together a complete service kit for a thermostat replacement on 1999-2003 Mk. IV TDIs. Often times, the thermostat housing breaks upon removal of the thermostat, or a bolt gets lost...

www.idparts.com

If so, with this kit, I won't have to drain and refill the coolant?

You got to drain the coolant regardless, t-stat is pretty low on the block so you will spill a gallon at least trying to swap it, might as well drain it, there is a drain plug on a lower radiator hose, you can reach it from the top

 

[BobnOH]

I read a thread recently about using a vacuum on the oil fill cap to remove a drain plug without draining. This might work with coolant as well. Otherwise you'll want to drain.

 

[MATPOC]

I read a thread recently about using a vacuum on the oil fill cap to remove a drain plug without draining. This might work with coolant as well. Otherwise you'll want to drain.

You got to be seriously lazy to rig up a vacuum instead of just getting a drain pan... and not gonna work on the coolant anyway, when you pull the t-stat you end up with 2 large openings