P2138 - Throttle/Pedal Position Sensor/Switch

Update on the Saga

programming DTC codes off using ECUflash and Tactrix 2.0 cable is possible, so I tried programming off p2138 code and reflashed the ECU.

The programming took hold and did what it said, and the car was driving, and boom! Car went into limp mode after a short while... and (as promised) with the p2138 code programmed off, the "check engine" light did not light up blinking. However, the cruise control light still came on solid, and the car still went into limp mode. In other words, the fault of a p2138 when detected by ECU is serious enough to still make the car go into limp mode, whether or not the DTC has been programmed off.

Other update: I got a spare ECU off ebay for about $100 and I reflashed it with my stock ROM from my original ECU. When you do an ECU swap on 2006 Legacy the car is undriveable until you get the keys re-registered to the car by a dealer. So I would suggest doing the swap AT THE DEALER rather than having to tow the car in...

But I circumvented all that by moving over the little 8 pin EEPROM from my original ECU to the replacement ECU, by unsoldering / soldering it in place. This carries the key codes over to the new ECU so re-registering keys is unnecessary if you do this.

So what did all this get me?

A working car again.

Been driving for days, and the random p2138 and p0139 codes have not come up since.

It was a bad ECU in my case causing the p2138. This is "worst case" scenario for this code. And would be the last resort looked at by dealer.

Man what a goose chase!

That would have been an expensive process if I let a mechanic / dealer follow the diagnostics to arrive at the same conclusion and dealer finally having to drop a new ECU in. They most certainly would have put a new pedal in first too. Probably would have added up to the value of the car.

So far, I have ordered a new TPS only to find out that I have the model with the TPS mounted directly to the throttle body. I've also disconnected and sprayed electrical contact cleaner on the connector for the pedal. My next move is to send back the TPS I don't need and see if I can take the cover off of the throttle body TPS and clean that up, assuming I can take off the cover (clips vs. rivets). Quick question there though, should I use the contact cleaner there or just a can of air to clean it?

Some additional info besides the symptoms already mentioned (limp home, etc...) The car has a rough/low idle (<1000rpm) after a min or two of warming up. Also, the accelerator, again only after a few min after startup, is unresponsive when pushing on it slightly. It actually loses rpm at one point before it starts to register that I'm pushing the gas pedal and start gaining rpms. Also, it starts very rough and will take 4 to 5 seconds of rolling over to fire the motor.

I'm not sure if those additional symptoms point in one direction over another, but I think, based on what I've learned so far from all of you, is that I either have a TPS on the throttle body problem, or a accelerator pedal sensor problem. Not sure what to focus on first.

Thanks in advance for your help/feedback.

So far, I have ordered a new TPS only to find out that I have the model with the TPS mounted directly to the throttle body. I've also disconnected and sprayed electrical contact cleaner on the connector for the pedal. My next move is to send back the TPS I don't need and see if I can take the cover off of the throttle body TPS and clean that up, assuming I can take off the cover (clips vs. rivets). Quick question there though, should I use the contact cleaner there or just a can of air to clean it?

If your throttle body was like mine, it was riveted shut. I drilled out the rivets to clean what was inside. This sensor uses hall-effect elements that are essentially hermetically sealed. So there was nothing to clean inside the sensor cover on this throttle body because there is no wear item. No dirt was in there to speak of. The only thing to clean out are metal filings that you put in there when drilling off the cover. And of course, there's a matter of putting the cover back on. I used several small bolts and a dab of glue on each nut after tightening down to prevent them backing off.

Some additional info besides the symptoms already mentioned (limp home, etc...) The car has a rough/low idle (<1000rpm) after a min or two of warming up. Also, the accelerator, again only after a few min after startup, is unresponsive when pushing on it slightly. It actually loses rpm at one point before it starts to register that I'm pushing the gas pedal and start gaining rpms. Also, it starts very rough and will take 4 to 5 seconds of rolling over to fire the motor.

 

I'm not sure if those additional symptoms point in one direction over another, but I think, based on what I've learned so far from all of you, is that I either have a TPS on the throttle body problem, or a accelerator pedal sensor problem. Not sure what to focus on first.

 

Thanks in advance for your help/feedback.

These additional rough idle symptoms could be many things. You need to get codes and/or operating parameters off the car to see what is causing the roughness. You can do simple things, like unplug the MAF sensor. This will throw codes of course, but also causes car not to rely on MAF so it enriches the mix and you can drive the car, using more fuel than you should... but it forces Open Loop - and the ECU ignores many inputs from emissions sensors. It can change your idle and responsiveness to pedal. It can change the starting (cranking) behavior. When you drive this way, maybe there is no "dip" or worn spot on the pedal ... maybe there's smooth revs up through that area. When you're done experiment, connect your MAF again. Clear codes. Are you getting front O2 / AFR codes? Car depends very much on a good front sensor for good running.. if you haven't changed that in 8 years or so, maybe it's due even if it is not coding. Rear O2 codes? Is the cat getting plugged?

The car can do limp mode for different reasons, not always / only the p2138 code. But if you pulled that code, then start with the pedal... work your way back to the ECU. Follow the diagnostic for tracing values on the pedal wires..

Thank you Scott -

Appreciate all the work that has gone into a greater understanding of DTC p2138 . . which you've documented to-the-nines for all the Subi community to see.

The infamous Limp Mode surprise made a white-knuckles introduction mid-point of an intersection. .. The Frau about had a heart attack. .. The DTC p2138 has struck twice (14 days apart) to our original MY05 Outback XT w/5EAT gearbox. ..

Best get with it - having a look at the connectors . .

Hello everyone.

My Brothers 2004 subaru legacy did the same thing and my wife has the same model. I looked for a fix read forum after forum. One thing I didn't read was people checking the three main plugs from engine to firewall. So I cleaned all of them and applied dielectric grease Now this was after I had cleaned all the other with no luck. He has been driving the car for 7 months now and the problem hasn't came back.

Thought I would share after a few moths trial. I hope this helps.

Hi,

I just wanted to post quickly my experience and fix for these issues.

I have been living with the occasional limp mode and P2138 code for about 2 years. Usually in cold weather, and heat down to the floor would aggravate it. Idle was never consistent, and throttle would often be all over the place. I cleaned the throttle body in the past with no luck, but decided to take the pedal apart a week ago.

Its two 14mm bolts to remove the pedal assembly (and the electrical connector), then 2 Philips screws to separate the electrical sensory portion from the mechanical pedal. See photos.

I cleaned all the contacts with acetone and gave them a light scrape with a scotch pad. While in there, I noticed that the board with the resistive traces on it was not firmly affixed. There is nothing by design to hold it from wiggling around...and I think that's where the problem was. I scraped the metal plate a bit, then glued the board down where it should be. Reassembled and reset the ECU.

A week later, it is running as it should

Hope that helps someone.

Cheers.

Circleback. Had it happen again before having a chance to look into it. Ripping up the Altamont Pass in the third lane and slap! No throttle car running at idle. Lucky I have the access port mounted in the car so a quick reset and back to rolling...

FF to what I've done so far. Simple as disconnect the harness connection from the pedal push some dielectric grease in the female side and reconnect. Also repeated disconnect and reconnect a couple times to be sure any corrosion present was cleared.

No recurrence after several drives. This one connection had never been removed in the lifetime of the car. Relatively dry environment so it's interesting this voltage drop can trigger a CEL, so sensitive. Curious if this sticks as a fix long term.

TB side of the equation has been off a few times over the years, so aside from sensor failure this tip of cleaning up the pedal connection is on point.