I have been getting an occasional P0125 code and after researching it yesterday it sounds like my ECT sensor is likely going bad. My MPG is down from last summer (about 24 MPG) and I assumed it was just winter weather. Its been running about 19-20 MPG but yesterday it was 15.21. We have been enjoying a nice cold snap in the negative 30s Fahrenheit so that would have some effect on MPG but it doesn't effect our 98 Forestor this much.

I'm planning on dropping by Napa this afternoon to pick one up. Any suggestions on what Brand I should aim for? I notice there is also a Temperature Sender Switch. Is that the same thing?

Brrr, I say...BRRRR!

Cold's coming to our 'neck of the woods' but only 0°.

Oh yeah....CTS....there are TWO related parts. One is a  2 wire (?) connector for the ECU/ECM's use (the one you want) and the other is a one wire sensor (?) that tells the dash gauge how its doing.  I've bought the NAPA part and it worked fine. 

Usually, sometimes, not always, the car will start hard /run rough when the CTS is bad - not just poor MPG. 

Do y'all put cardboard or some kind of shield in front of the radiator to 'keep it warm'?

2 minutes ago, wtdash said:

Brrr, I say...BRRRR!

Cold's coming to our 'neck of the woods' but only 0°.

Oh yeah....CTS....there are TWO related parts. One is a  2 wire (?) connector for the ECU/ECM's use (the one you want) and the other is a one wire sensor (?) that tells the dash gauge how its doing.  I've bought the NAPA part and it worked fine. 

Usually, sometimes, not always, the car will start hard /run rough when the CTS is bad - not just poor MPG. 

Do y'all put cardboard or some kind of shield in front of the radiator to 'keep it warm'?

Thanks! So my car starts normally and engine temperature comes up nicely and is normal. Just the code and the poor mpg are the symptoms.  This sensor is the one under the Idle Air Control valve on passenger side of engine?

Some people (not many) put a piece of cardboard in front of their radiator. Those people might just need a new thermostat. Our cold snap is coming to an end this weekend. It was only -26 this morning at my house. Sunday expected high is -22 and Monday -7. 

The CTS doesn't have anything to do w/the dash gauge sensor. There is/was a Youtube about them. Yep - should be on the passenger side - under all that other stuff.

Tangent: Actually easier to access on an EJ22T as they're on the driver side.

So, it could still be the CTS going bad. ...but I'd also suspect the O2 sensors.  The ECU runs in 'open loop' (pre-defined parameters) when car is started and doesn't 'listen' to the O2 sensors (if I remember my tuning info from years back), and then run in closed loop  after it's up to temp (ha, ha - good luck) where the O2 sensor tells  the ECU what's going on in the exhaust - incorrectly if old/worn out -  and could then be causing poor MPG.

26 minutes ago, wtdash said:

The CTS doesn't have anything to do w/the dash gauge sensor. There is/was a Youtube about them. Yep - should be on the passenger side - under all that other stuff.

Tangent: Actually easier to access on an EJ22T as they're on the driver side.

So, it could still be the CTS going bad. ...but I'd also suspect the O2 sensors.  The ECU runs in 'open loop' (pre-defined parameters) when car is started and doesn't 'listen' to the O2 sensors (if I remember my tuning info from years back), and then run in closed loop  after it's up to temp (ha, ha - good luck) where the O2 sensor tells  the ECU what's going on in the exhaust - incorrectly if old/worn out -  and could then be causing poor MPG.

Last summer I occasionally got an O2 sensor code so I replaced the upstream O2 sensor and haven't had any more O2 sensor code errors.

Yep, if that's the only code, you likely need the sensor. 2-wire sensor, aftermarket listings usually have the one for the gauge listed as a "switch" and for the ECU as a "sensor", but that can vary.

47 minutes ago, Numbchux said:

Yep, if that's the only code, you likely need the sensor. 2-wire sensor, aftermarket listings usually have the one for the gauge listed as a "switch" and for the ECU as a "sensor", but that can vary.

Thanks!