Piston Slap: An STI With A "C. Diff"???

TTAC Commentator robstar writes:

Hello Sajeev+Piston Slap Audience,

A few weeks ago I saw something strange with my lightly-used-never-abused 2005 Subaru WRX STi — A “R. Diff Temp” started flickering on the dashboard. This particular day it was quite cold and it came on as I reversed out of the garage backward down a sloping driveway. After going one block (in forward, not reverse) with this behavior, I turned the car off and let it sit for a minute. After turning the car back on, this didn’t happen again for about 3 weeks — until after the mechanic changed the oil. I drove the car back to the mechanic who looked under the car & thinks the sensor is going bad (7 year old car, 45k miles, ~ full synthetic oil change 3 times/year). He said he can’t see any sign of an Rear Diff leak so it should be OK to drive home (40 miles). After the first 7 miles or so mostly city the light turned off & was completely off for the last 33 or so miles all the way home. The only prior issue I’ve had with this car was an engine valve spring at 10k that was replaced under warranty.

I’ll admit — I’ve been a bad on the maint side, however this car sees about 80% highway usage typically at 55-65mph. No clutch dumps, no redlining — rev matching when downshifting……I pretty much baby it. So why did I buy this car? For fun in the snow.

I am going to have him actually swap out the R. Diff fluid when he replaces the sensor, but do you guys think it’s simply just a bad sensor or some other problem?

To make things easy, the car has NEVER been modified (Yes, it’s probably the only Stock WRX STi that exists). The only parts different from stock are that I now have winter tires (stock size) on steel rims (stock size) for the winter here in Chicago.

Thanks Sajeev!

Sajeev answers:

Is there a chance that, during the fluid change, the sensor was (unnecessarily) removed? And over-torqued? And it broke? Oh wait, the “after the mechanic changed the oil” remark was about the engine, not the differential. Right?

Sensors can go bad at any point, that’s the beauty of a somewhat fragile/complex piece of electronics living in a rather brutal environment. I can’t Google a good reason for why your sensor is out of range, so maybe the control module is at fault. Or maybe the ground is dirty and no longer doing its job. Or the wiring harness is damaged somewhere. This thread on NASIOC looks pretty comprehensive for your diagnostic needs.

So there you have it. Good luck with all that.

EDIT: A far superior title came to me, thanks to the B&B. Explanation here.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.