2001 Forester Code PO172

This is on a normally aspirated car and the CEL just came on and has stayed on. We did have it come on about a month ago while driving over a mountain pass over 9000ft but it went off again.
The car lives at 5000ft normally and has to get over the pass every few weeks when my daughter comes home from college
The code description says "Fuel Trim Malfunction (A/F too Rich).
I can understand that air is thin at 9000ft and could cause a rich mix, but i would have expected the computer to adjust for this.

The guy at O'Reilly who read it said the O2 sensor may be blocked.
Assume he meant the upstream one
Has any forum member had a similar problem and can give me some advise on what the fix might be?
many thanks
Ian

P0172 is usually a metering issue. The front O2 sensor is an air/fuel metering device. The odds are this is the culprit. Running too long with a rich/lean condition can result in cat converter issues, so keep this in mind. Good luck.

Thanks for the insight.
The CEL went off again, but i assume it will come on the next time she goes up the mountain.
When it warms up a bit I'll get a new upstream O2 sensor installed.
Is there any kind of airflow sensor pre-combustion?
Seems not on this model but looks like there is on later Subaru's
Ian

First clean the MAF(mass air flow) with good electronics cleaner or brake cleaner, not carb cleaner, something with no residue left behind. Sometimes the MAF can trick the O2 sensor into appearing bad. Then verify your O2 sensor with a volt meter. You can look on Youtube or Google how to check it. Intermittent CELs are tedious, but I think one of these two is the culprit.

Interesting solution. My daughter reported banging from the underside. Turns out is was actually an exhaust system leak. The bracket from the Cat to attach to the frame of the car was broken causing a small home in the cat. Had a new bracket welded on and no more problems.

Just goes to show that getting a code, doesn't always tell the whole story. Makes sense though, small hole in Cat tricking the O2 sensor and making it appear bad. Glad you got it fixed.

Not sure I should be adding to this thread, but the stuttering when climbing hills at altitude is back.
Checked for holes in the cat but none found. This time no CEL, so nothing to tell me what it is.
Anyone had this with a 2001 Subaru Forester?
The latest time was at about 6000ft when going up a long hill. My daughter also tells me it happened when she was crossing the Sierras at about 8000ft plus.
Wondering if I should be suspecting the O2 sensor, if so which one, I think there are 2?
Car has about 165k on the clock
Thanks
Ian