vert

Just picked up a 2003 c320 sedan with 80K miles and had to replace the alternator right away. While test driving the car after the alternator replacement I noticed that sometimes I would press the gas pedal and the car would hesitate for 5 or so seconds before building up RPMs. This would happen when driving or when parked. Turning the car off and on would temporarily resolve the issue and the gas pedal response would be normal for some time.

I pulled the codes and one code is pending which is P0123 Throttle/Pedal Position Sensor/Switch "A" Circuit High. CEL is not on yet in the car as this code is still pending. However, the car definitely does not react to the gas pedal as it should all the time. The delay when it happens is long enough that other cars will pass me from a stop light and i would hold up cars behind me for a couple of seconds while my car builds up speed.

I searched this forum and found one other thread but it was not specific to P0123 code. What would you recommend in terms of troubleshooting this problem?

vert

Searching for TPS instead of the code brings back a bit more info. This one describes the same exact issue I am having: https://mbworld.org/forums/c-class-w...s-failure.html. How is everyone solving this issue - buying a new sensor?

pcy

Try to RESET the throttle first. Follow the following steps:

1. Turn the ignition key to the on (NOT start) position.

2. Press the gas pedal to the floor and HOLD for five seconds.

3. Turn the key to the "off" position (don't remove the key), then release the gas pedal.

4. Wait at least two minutes - don't do anything.

that should reset the throttle position sensor. Please try the above and report back.

vert

Thank you, that's exactly what I did yesterday after finding the procedure below and it seems to have cleared the P0123 code for now. I started the car a couple of times to see if the pendingn code would come back but so far so good. Car revs fine too. I am wondering if the failing alternator caused the TPS sensor to put out incorrect values? Really hoping that this reset is a permanent solution and the code does not come back. I will drive the car and report back.

Put key in: 1 click: 2 click: Start
This is legend follow below this.

Put key in the ignition
Turn once (click 1)
Press pedal all the way to the floor (hitting the quickdown)
Hold for 5 sec
Turn the key to off position while holding the pedal all the way down
wait 1-2 sec, Let go of pedal and wait for 2 min for ECU to reset.

DO NOT TAKE OFF THE KEY WHILE WAITING FOR 2 MINUTES

EDIT: I see that you edited your post to include the procedure. The only difference is that I turned the key ONCE after inserting and yours is TWICE untill all lights go on the dashboard. Not sure which one is right or if they do the same, but the one I used worked for now.

pcy

Key should be in the 2nd position (the stop BEFORE starting the car).

vert

Cool, I'll do both ways... won't hurt.

Now another small issue is the driver's side head rest that is stuck in the lower middle position. When I use the electric switch to move it up and down, the switch makes clicking noises but the headrest does not move at all. I tried holding the switch up for a while and then down for a while but that does not help. Searched but did not come up with the same problem. Any idea what else to check to save posting a separate thread?

pcy

Only the headrest part is not working, all other functions of the seat are working, right? If so, the small motor that moves the headrest may be dead. Before we jump to conclusion that the motor is dead, you should open the switch module on the door and clean the contacts. It's possible, the contacts may have rusted.

vert

Yes, all other seat functions work just fine. Is there a diy on how to open the switch module? Do I need to take the whole door panel off to get to it or can I just open the switch from the outside somehow? Looking for specific directions as I do not want to break any more clips etc.

vert

The TPS problem came back after driving a car for about 10 minutes. Same symptoms i.e. no acceleration when you mash the gas pedal. Reset the ECU again and the problem went away, but obviously it is only a temporary solution. So what's next? New sensor? Where is it located? Any DIYs? Where to buy?

Glyn M Ruck

Replace the pedal assembly unless you can find the sensor & fit it yourself. The sensor is glued in place on most cars.

vert

Any suggestions on where to get the pedal assembly or the sensor by istelf (which would be better) besides the dealer? Tried searching a few places but no luck.

Glyn M Ruck

RMeuropean have the pedal assembly in stock for $174

The sensor is not commonly available but somebody local might have found a source. Hope it's not the throttle body because that is real expensive.

Most likely the TPS though.

vert

Followed this advice, purchased a new assembly from a Mercedes dealer that matched an online price, installed the new pedal, cleared the codes with a scan tool and everything has been great for several weeks now. Your help is much appreciated!

I posted a new thread on the headrest issue... I'll get this car all sorted out!

azhia.janay

I know I'm two years later but which pedal assembly did you replace was it the parking break pedal assembly?
Like this one: http://www.rmeuropean.com/mercedes-parts.aspx#Category

vert

I replaced the gas pedal assembly. Still no issues 3 years later.

Josh Sall

Robert Newhook

Hey Josh,
Did you ever end up fixing your issue? I'm planning to buy the pedal position sensor (PN 0125423317) because I'm having the same issue.
The car drives fine as long as you don't floor the accelerator pedal. The minute I floor the gas, the throttle response instantly becomes significantly slower.
Best,
Rob

ccozic

Today, this method DID work for me:

Put key in the ignition
Turn to second position (before Start)
Press pedal all the way to the floor and hold 5 seconds.
Turn the key to off position while holding the pedal all the way down
Wait 1-2 sec, Let go of pedal and wait full 2 minutes before starting
DO NOT REMOVE THE KEY WHILE WAITING FOR 2 MINUTES

Couple days ago my 2002 C320 automatic was slow accelerating from a stop. Then it went into limp mode, Check Engine light on and could not get out of 1st gear. I came to a stop, motor running, and it would not go into Reverse. Drove the side streets back home in first gear and my Engine Overheat warning light came on. I turned off and let it rest a half hour, then drove a few blocks back to my street, still only in First.
I tried the above method, but held the gas pedal down for 20 seconds, then waited a full FIVE minutes before starting. It got out of First gear, but would not shift out of 2nd. I came to a stop, engine running, tried again and it would not shift out of First.
Today I modified by holding pedal down just FIVE seconds, then waiting TWO minutes. I drove off, CEL still on, but it did shift from 3rd into 4th. Came to a stop and turned motor off. Started, drove off, and it shifted out of 3rd. Came to a stop, motor running, drove off and it shifted out of 3rd. All the while CEL still on, but the shifting now works and I can put in Reverse while car is on.
Will have to pull codes and see what's what: low tranny fluid, vacuum leak, or ??

StugartsCLK

Do you happen to know what model/parts # that assembly was?