The Goodyear mechanic is telling me that this p0170 fault is caused by the rear o2 sensor. The first mechanic I took it to said it was the front o2 sensor. I replaced the front one but the fault came back. I took it back to him and he thought it might be the EMC. I thought he was guessing so I took it to another mechanic. Does this second mechanic's conclusion sound right?

Anyone home?

The average voltage taken from the rear sensor is logged at the conclusion of each driving "event" (engine start, to key off) and is used to influence fuel trim.

 

Therefore the exact problem could be anything that influences the rear sensor's average logged value to such an extent that it drives the trim parameter out of range. e.g. defective rear sensor, defective connection to the rear sensor, or an exhaust leak located after the first sensor but located before the second. I can't think of any more, but I'm sure there are more.

You don't say what model Subaru you have . depending on what model it is there may be a recall on it for that code

 

SEA#3

It's a 1995 Legacy L Wagon Mfg date 1/95.

I remember having a fuel trim malfunction code on my 99 Forester. It's one of those annoying codes that can cover a large number of components. I tried a number of measures to clear the code, without luck. I had just about decided to live with the problem when a recall notice from Suabru appeared in the mail. When the MAF sensor was replaced with code magically cleared.