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///M3 Forum
The BMW E46 ///M3 is the M version E46 and puts out an amazing 333 HP and 262 lb-ft of torque at stock specs! There are an amazing amount of modifications for both the coupe and convertible models so read up and get started modifying your cars today!

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Old 10-29-2008, 01:33 PM   #1
Registered User
Join Date: Feb 2004
Location: SoCal
Posts: 39
My Ride: E46 M3 Conv
Got an EML & ASC Light + Limp mode? Fix to the TPS sensor (+pics)

What happened:
I was driving home and while the car was decelerating following an acceleration, engine power cut out and went into limp mode. "ASC traction" light turns on (the exclamation mark with a triangle around it) as well as the "EML" light and "Check Engine" light. It seems that only about 20% engine power could be used.

I pulled over and restarted the car and after driving a few miles, the same thing happens again. Doing it again, it started occurring more frequently at which point I got home two stops later.

What you need:
  • OBD-II Scanner
This is a MUST-HAVE in most garages! The dealer will easily charge you double the cost of ONE of these just to read your codes once. The scanner also works in nearly any car made past 1996 since the system is largely universalized. If you're in SoCal, PM/email me if you need me to read your code.
  • Multimeter
Something that can be used in far more than car parts, I would highly recommend picking up one of these too. You can get them for pretty cheap online. They measure electricity flow and continuity on pretty much any application you can think of.
  • Philips head-bit, socket wrench set
  • Flathead screwdriver (not necessary)
  • Angled Pliars (not necessary)

Use the OBD-II scanner and PULL THE CODES. Without the proper codes, you won't know what's wrong with your car - it has sensors and diagnostic checks to make sure that it lets you know what the problem is. When I pulled the code, I got "P0121 TPS Sensor." What a TPS sensor is a Throttle Position Sensor which measures where the throttle is. I also got a few other TPS codes and a mixture issue, possibly caused by a number of reasons as the car stalled and such during this ordeal.

Background Information
To understand what is going on, the easiest way to do it is to picture it in your head. You have a three TPS sensors in the M3, because it's a fancy "drive-by-wire" system, there is no actual hard line that runs from your gas pedal to the engine as in most cars. It sends an electrical signal to the engine to tell it how much you are pushing down the pedal. A TPS sensor is mechanically something called a "potentiometer" which is a fancy way of saying a switch that tells you how much you are pushing it down. Read more about it here if interested:

Because of this "drive-by-wire" system you have three potentiometers in your M3. One is your pedal, which has to measure how much you push the pedal to give off a signal. And the other two I am not sure about, but theoretically I assume one that measures how much the throttle body opens (the front one) that is hooked up to the throttle on the engine, and another one that is apparently behind the intake or something. Anyway, according to other threads, it's less common when that one goes out so we'll focus on the first two and if it doesn't fix your're boned, have a nice day.

It's pretty simple. For the front TPS sensor, you take your philips head bit and socket wrench to unscrew the two screws and pull the TPS directly toward the front of the vehicle. There's an O-ring and the circular aluminum covering also may pop off, so just keep your other hand under it when you pull it off.

For the gas pedal, from kneeling position you'll see a white tab and on the right side (against the firewall) that you can't see, there is a sensor and connection there. Disconnect that link by hand, but it also helps to have angled pliars. Then you push the white tab and pull the pedal sideways toward the brake pedal. It should slide right out.

Now with most TPS sensors, they have a given range. I watched this video on YouTube and they say it should normally give a 0.5 ohm reading when you aren't pushing the gas pedal and 4.5 reading when it's floored.

Put your multimeter on 20-ohms and connect the two probes to two of the three pins until you get a reading (usually outer and middle). Measuring my FRONT TPS SENSOR I got 0.93-ohms and 5.56-ohms. If it moves COMPLETELY SMOOTHLY then it is NOT BROKEN. However, if there are spikes on the ohmmeter while you rotate the little white thing inside, then you can go to and overnight the sensor so you can get to work the following day (call your boss to let him know you're sick tomorrow). Mine was completely smooth, so I didn't have to bust $130 on this part - price according to other threads.

Next, was the gas pedal TPS sensor. There are six leads and to be honest, I don't remember which ones I used, but along the first row of three pins, try two of those and if it reads 0, try the other two. Same with the second row of pins. It helps if you have a steady hand and a friend to either hold the multimeter's probes or push the gas pedal down. Mine had a spike while pushing down the pedal. In fact, it was all over the place from 0-ohms to 4-ohms and all the way to 12-ohms or so. My gas pedal is f'd. I overnighted a new gas pedal from Tischer BMW and it came the next morning. Right under $160 shipped across the country. My local BMW dealerships wanted $175 + California tax (8.25%) and a 2-3 day wait since none of them had it in stock.

My old one was made in 02 and the new one in 08. Little difference except the new one is like an ounce lighter, can't stand by itself on a desk (weight is off?), and doesn't have the rubber bushing shown in the picture below.

Installation is easy and just reverses the way I took it out. With the new gas pedal, I've noticed the following results:
  • Better throttle response
    Less jerkiness (the car & me)
    Smoother shifts
    No engine-cutting out on the freeway when I pop it in neutral to roll down a big hill
    Easier getting into gear 1 and 2

My Theory
When I think about it, it kind of makes sense - when the potentiometer can't give a smooth signal, it sends a spike to the ECU and when throttle used to be at 20% going to 25% and suddenly spikes to 80% for a split second, the car freaks out. The engine usually follows along for a while, hence the jerkiness in my power delivery in the past. When enough of the TPS/potentiometer is messed up, it just sends garbage signals to the ECU and thus the ECU says "I'm not having any more of this ****" and puts you on limp mode and only gives you a little bit of power so you don't feel like you're driving a mechanical bull. Then it waits until you fix it.

Well, I hope this writeup helps somebody. It was a bit stressful of an ordeal, but through a diligent diagnosis and researching a bit about the situation, I probably saved myself $600 of labor at the dealership and a couple days of my life without a car.

Special thanks to the other people on this forum. I'll post quotes that I found helpful.
JonHsiung is offline   Reply With Quote
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