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June 08, 2014


I'm still working on my 2004 Subaru Outback, this time attempting to change the engine temperature sensor. You'd think it'd be a simple job, it isn't.

You may want to refer back a couple posts to where I pull the engine and replace the head-gaskets, timing belt and components and the clutch. There are followup additions as initially the new rear main seal began to leak and I had to pull the engine again and replace it. It's not leaking now and the car is running fine.

However; Every 400 miles or so an engine warning light trips and the error relates to the O2 sensors, bank one, catalyst below efficiency. This happened before the rebuilt only then it was within 100 miles or so after resetting the computer.

I read where a faulty temperature sensor could cause this problem, along with leaking heads... well the heads are fixed, they are no longer leaking after replacing the gaskets.

So I bought a new temperature sensor for the engine. It's a Well's SU4300. This actually goes back to my original post where I begin to replace the head-gaskets, as one of the first things I did was to replace the temperature sensor, then I did the heavy work that followed.

It took me roughly a week previously to do all the work - so imagine my shock and awe when I'm finished with the big-job and I start the car for the first time? It started right up and ran well. But after a short drive I noticed the temperature gauge was in the red and rising. I scooted home.

The engine didn't act like it was over-heating. The lower hose from the engine was cold, indicating the thermostat which was new hadn't opened yet. I worried some about a vacuum block, in the coolant, but given the obvious I decided against that idea. I revved the engine a a few times, and waited. The lower hose finally got hot, pretty much when I expected and the cooling fans came on when expected also, lasting less then a minute which is normal.

The engine temperature was normal therefore despite what the gauge read. So the only thing I did was put the old sensor back in. And the temperature was back to normal (I waited several hours since the coolant was hot).

Recently I bought another temperature sensor from another company after getting a refund from the first. It was the same model sensor, a Well's SU4300. I noticed after installing this one the temperature of the engine read normal operating temperature and mid-range even before starting starting the car. The engine was stone-cold, it should not have a reading already. Again, after starting and running for ten minutes the gauge went above the red-line, indicating over-heating.

This is a picture of both the old and new sensors:

A slight difference in design is to be expected.

So now I have a dilemma. Two new sensors, from different parts stores both defective??? Same manufacturer. But a reputable one. Same part #. Yet the old part is the only one that works and reads the correct temperature. You'll be told to buy OE parts from the dealer, and in some cases that's the only place you'll find the part, but for something like a temperature sensor it is very unlikely only a Subaru replacement will work. The auto-parts stores would go out of business if such poor and defective parts were being sold.

So, since I don't have the answer to this mystery, and the clerk at the parts store doesn't, I signed up for a free-3 Day subscription to JustAnswer,com, with a Subaru Expert apparently standing by. I have not yet received my answer. When I do I'll add it onto this post.

Wish me luck.

UPDATE June 10, 2014

I learned that when you ask a question don't merge two questions into one. The tech who answered my question was sharp though. The two questions I asked of which this one he answered really wasn't posed as a question, was, that I was changing the temp sensor in hopes of resolving a "Catalyst below efficiency" error. The real question was why do two new sensors, the only one (I found a different one, an OES which, the last time I ordered an OES part it was a genuine part, a Toyota part.) sold, as I was saying, wig out both times? What he did answer was the exact P0420 code, that I hadn't mentioned. He said 95% of the time that means a bad CAT... being BobKat, I'm focused on the 5% as the CAT is just over a year old, installed by a reputable technician. And I suddenly found a picture of my exhaust system indicating the 5 O2 sensors, of which I change the very front - 2, and the very rear - 1. I didn't change the center ones after the primary CAT, and that I found out is Bank 1. That's what the code says is in error.

These cars though - confusing??? I have either an EJ 253 (of which most of my parts were - including the head-gaskets which are holding up really well) or the EJ259. Not only is it hard to find enough information to know for sure but the EJ259 is the PZEV - High Efficiency engine, but no where does mine say that, though I know it's CA emissions. If I do have a EJ259 I may have installed the wrong head gaskets but I carefully compared it and the holes to the old one, and found no differences other than a better design.

And oh yes, I found an OES brand temperature sensor - another $20, last time. And plan on replacing those center O2 sensor when I can. I'll bet a new CAT is around 2 grand. I don't dare look!

UPDATE: Saturday, June 14, 2014

Success!!! AutoPartsWarehouse had the temperature sensor I needed. I ordered it Wednesday night - the text listed said there would be a 3 - 4 day delay, but I received it that Friday! They label it as OES, but as you can see from the packaging, it is a Subaru part:

And it did the trick. I installed and let the engine warm up and it registers perfectly the coolant temp, actually a bit lower than the sensor that had been in there.

Lesson to learn... the Wells SU4300 may look like the right part, but it's not. I base that on the fact I installed two of them from different retail sources, and neither worked. Only the OES part worked.

UPDATE: The P0420 Engine codes were a result of one of two bad O2 sensors in the central part of the CAT. One side had been changed, the other (bad side) hadn't.  


  1. I have been taking tha same journey as you. I just changed the coolant temp sensor with an autozone one. I noticed right away that the gauge was at the half way point which was odd on a cold engine. Luckily I had my meter telling me what the actual temp was which was not hot.

    Did you ever figure out what the p0420 codes was from?

  2. Where is the ECT sensor located on 2004 Subaru Outback

  3. Where is the ECT sensor on 2004 Subaru Outback