symptoms of thermostat failure

Can anybody tell me what the symptoms are of a thermostat failure? Curious to know as there seems to be some talk about the thermostat eveyr now and again. What I mostly read is that it fails open/closed. Can someone explain exactly what they means?

If your thermostat fails, the Service Engine Soon light will come on.

They're designed to fail open; this keeps coolant flowing. If they failed shut the engine would almost immediately overheat. In addition to the SES, if it does fail open, you won't have much heat from the heater.

Originally Posted by floydarogers

If your thermostat fails, the Service Engine Soon light will come on.

They're designed to fail open; this keeps coolant flowing. If they failed shut the engine would almost immediately overheat. In addition to the SES, if it does fail open, you won't have much heat from the heater.

Further to the OP's question, there are other things which are "failures" of a BMW thermostat. One of the more common ones is for the electrical resistance heater to burn or short out. Most stats open solely due to engine coolant temperature, but BMW will open theirs sooner and faster by heating them up with a signal from the PCM. That happens whenever the PCM weighs air temperature, coolant temperature, accelerator pedal position, and engine load and decides it needs to get the cooling show on the road right away. Then it zaps the stat, heats up the wax pellet which causes it to expand and force the stat to open.
Failure of that circuit wil usually set a thermostat specific code.

Hi all.

I replaced the thermostat yesterday after the temp guage on the dashboard was at about 30%, running cold, Suggesting the thermo was stuck open, Also the heating inside the car wasnt great, further suggesting the same problem.

The temp guage went back to 50%, (12 o clock) which is normal and the heating returned to normal. Quite hot.

However, this only lasted for a while and now the problem of low heat inside the car and temp guage has returned to about 30%.

Does anyone have any experience of this oir any advice?

Note I only changed the main thermo.

Thanks in advance

Originally Posted by Daithi

....now the problem of low heat inside the car and temp guage has returned to about 30%.

Does anyone have any experience of this oir any advice?

Note I only changed the main thermo.

What codes are in memory?

two codes showed up in memory. But Im told they were to do with a sensor for the air bag. I think one of them was a code 51? does this make any sense?

Originally Posted by Daithi

two codes showed up in memory. But Im told they were to do with a sensor for the air bag. I think one of them was a code 51? does this make any sense?

No....ask for a complete readout including hidden & shadow codes.

Alternatively