emt1581

The problem stems from having only 2x sensors on a critical system (e.g. my Subaru gas pedal) there is no tie-breaker, when one sensor/input goes bad. That, and very simple software for reading sensor inputs.

 

ECM software is checking for constant agreement of sensors. Even when the pedal is not being used (e.g. on cruise control) both sensors must transmit values that are in agreement with one another. A sensor giving a different 'answer' about pedal position than the other forces the ECM to make a choice "what is the correct pedal position"? It doesn't know which of the redundant inputs to trust anymore, so it abruptly gives up! ECM response to giving up is to put the car immediately into the safest compensating mode it knows how for "flying blind" (no longer knowing what the driver wants for speed).

 

The ECM cuts power the throttle. The throttle has a spring loaded return to a minimum open position... which would be your "fast idle" about 2500 RPM. Which I think everyone agrees is better than the throttle snapping shut (stalls the car), or throttle flapping around in the breeze, or a throttle flinging wide open (full power). It's simply the most responsible way of handling loss of the critical system. Given the limitations of its inputs, and its evaluation of them.

 

It would be better, of course, to invest more effort and $$ into the critical systems to make sensor failure less likely. But this is a car and not an airplane. Or a Mars lander.

 

Simplest thing: adding a 3rd sensor to the pedal as a tie-breaker. ECM would then use a 'majority rules' pedal position. Having sensors of different technology types would be an improvement so a common problem wouldn't knock out all of them. Three sensors would be a better match for the simple software Subaru is using in the ECM for this problem.

 

With just 2 sensors (like our Subaru pedals) good software could use many heuristics to deduce a sensor that is "acting up" or out of character, better noise rejection.. The ECM could consider, if not already:

- the quickest possible human speeds of pressing the gas pedal. (Reject samples from either sensor where quicker-than-possible pedal change has occurred).

- the (spring) constant of how fast the pedal can possibly move when released suddenly. (Reject samples exceeding this rate).

- data smoothing (averaging) instead of instantaneous sample points being used for difference comparisons.

 

.. the penalty here is some of these things take time to determine, which shows up as "lag" time between when you WANT a difference and press the pedal when your "request" actually registers and the car responds.

 

Tempted to connect your pedal the throttle with mechanical linkage again? Good ol' days!

In other words, this is the result of crappy engineering that Subaru intentionally did and won't (or maybe didn't....to give them the benefit of the doubt that future models are improved) spend money eliminating?

Is there any permanent fix for this short of blowing on the sensor every few months?

Thanks

-Emt1581

The p2138 code seems to get more and more aggressive / frequent over time.

 

Mine got to the point of about 1 per every drive cycle, before I stopped letting my wife drive the car. Too unreliable, too dangerous.

 

I started up my 1987 Mercedes 300D (fresh battery and one crank of the key fired right up)... after sitting for a couple years. Most reliable car on the planet... she drove that while I diddled with the code.

 

I think I solved p2138 finally by cleaning the sensor traces in the pedal... very careful operation, detailed in another thread.

 

http://legacygt.com/forums/showthread.php/bad-throttle-position-sensoriii-158048p3.html

 

 

My car is like yours, a 2006, and I've got 200,000km on it. That's a lot of back and forth wear on the pedal sensor. But it was possible to clean it up.

 

Been driving now for 3 x days not once thrown the code. So definitely an improvement!

I'll give it a look over the weekend. Thanks for sharing!

Still curious why Subaru would do such a dangerous thing like allow the sensor to trip the system and stop the car while driving in traffic!! That is law-suit territory!

-Emt1581

Wanted to check back in....sensor caused a check engine light again today. Basically killed the car when it happened. My wife was going around 5mph in traffic (coasting) and all of a sudden the check engine light came on, car wouldn't accelerate and she barely made it into a nearby parking lot.

Came home from work, hooked up the OBDII and got the throttle position sensor code again. Took out the pedal, blew in the male and female end, put it back together, cleared the code and drove it around for 30min. It's fine.

How is such a minor issue able to cause such a systemic/catastrophic failure?!?! I could understand the light. I can understand an alarm or something...but why program the car to completely shut down if a position sensor trips???

Thanks

-Emt1581

Been a few months....still fine.

 

The airbag light comes and goes which just means I need to work on or replace the overhead light board.

 

Other than that, no issues.

 

-Emt1581

Been 6 more months...still fine!

-Emt1581

give it a few weeks

Been a few months....still fine.

The airbag light comes and goes which just means I need to work on or replace the overhead light board.

Other than that, no issues.

-Emt1581

Update...problem solved!

Visited the local Subaru dealer and asked about diagrams/part numbers/ cost of replacement/etc. After talking to a mechanic the guy said he was told I could try to clean the terminals/sensors but usually once the check engine light came on it was time to replace. I was also told about the two 12mm bolts holding the gas pedal in place and the plug connecting everything. Finally, if a replacement was needed it would have been $180 out the door.

Came home, unbolted the gas pedal and disconnected the plug. Blew into it...boy was that full of dust!!, then I reconnected everything and started the car....no more check engine light!! Hasn't come back since!

Seems like it worked. And if it comes back, I can clean it again or just replace it but I'd find the part online rather than paying $180.

Just figured I'd share.

Thanks!

-Emt1581

The 06 Legacy I just bought my wife last Tues night had this code come up. Are there any vids online showing step by step how to dismantle/clean everything?

Thanks

-Emt1581