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Posted by: ricerrx7 Jan 17 2006, 10:34 AM

Many people ask about the ECU reset. What does it do? Is it really worth doing? Are there any negative effects? I think it's time to have a definitive guide to the ECU reset.

Why do we reset the ECU's?
The main reason we reset the ECU's in our cars is for more power. Plain and simple. Why? After a little while of driving, the ECU de-tunes the engine to satisfy the O2 needs of the cat. When you reset the ECU, it resets the fuel maps to a base value that's more suited to power.

Is this bad in any way?
Yes, it can be harmful. When you reset the ECU it changes the fuel maps, which is potentially harmful to the cats.

How often should I do it?
Only when you need the extra power. If you are going to race, dyno, or show off, these are the only times you should do the reset from a power standpoint.

How do I reset the ECU.
There's multiple theories about this. The time consuming way is to disconnect the negative battery terminal for 15 minutes, press the brake pedal, then reconnect the negative terminal. This method is fine if you want to wait around 15 minutes, and reset the clock, and the radio presets and settings. A simpler way is to pull the two fuses labeled "ECU" in the underhood fuse block. And the easiest way (after initial difficulty) is to hook up a switch; http://elantragtclub.tripod.com/elantra/id514.html


Any more questions? Feel free to ask.

Posted by: Cite 3 Jan 17 2006, 10:44 AM

when i show off? its almost as paintstaking as opening the trunk to open your nitrous bottle.

lol. i guess i should look into that Switch DIY huh?

as far as "power" i do it because when i add a significant modification, i dont want it acting all crazy.. so start from the beginning. yes, the fresh start up feels pretty good and response is like new.

Posted by: southpawboston Jan 17 2006, 11:51 AM

okay, since this topic comes up a lot, i'm pinning it.

Posted by: rickl115 Feb 18 2006, 02:52 AM

so what about if i disconnect only the positive. does it have the same effect?

Posted by: Seinster Feb 18 2006, 03:27 AM

The correct procedure is to disconnect the ground (negative) terminal from the battery. This will make it safe for you to work on the electrics without inadvertently causing a short circuit which could immediately burn out your wiring (or worse).

This may sound strange to you. Why disconnect ground? Wouldn't it be more logical to disconnect the voltage carrying (positive) terminal from the battery? Well, there is a good reason. First: electrically speaking it does not matter which terminal you disconnect. When the circuit is broken no current can flow. It does not matter whether the circuit is open at the positive or at the negative terminal. BUT ... When you loosen the bolt or nut at the positive terminal of the battery you will need to use a spanner. When turning the (steel) spanner it may easily touch a metal part of the car body. That will create a massive short circuit! In fact you have now turned the battery and spanner into an arc-welding set-up with all the associated fireworks. I would strongly advise you not to try this. However when you first loosen the clamp on the ground terminal you can have the spanner touch the body of the car without any ill effect. The only thing to look out for is that it does not touch the positive terminal of the battery.

As for waiting 15 minutes, that's BS. It can be done in 2 minutes, just press the brake pedal multiple times to get rid of the residual current.
To really know if you have reset the ECU via this method, you'll see your guages go crazy when you put your key in and start her up.

Important thread though.

Posted by: ricerrx7 Feb 18 2006, 09:32 AM

I'm not sure what you mean seinster, the guages in my car do not go crazy when I reset the ecu. Also, there's no need to wait 2 minutes and press the brake pedal if you just pull the underhood fuses or hook up the switch...

Posted by: Seinster Feb 19 2006, 12:14 AM

I didn't mention those methods, having never tried them...

Note also, there is no need to 'wait', I meant the procedure with removing the terminal and pressing the brake pedal takes two minutes to do.

Regarding your guages not going crazy, perhaps this is only on the XD2's. Not really sure though.

As soon as I reconnect the neg.terminal, guages go vertical, or else they do a full swing, then after the car is started everything is back to normal. Scared the **** out of me the first time I did it.

Posted by: Mobius1 Apr 2 2006, 05:28 PM

yeah can't say i've ever had my gauges do anything like that either but would be cool to see...well at least to me.

Posted by: DJ Hellfire Sep 5 2006, 08:20 PM

I have one question on something that wasn't totally clear in the switch DIY. So, exactly which pin # is the blue power wire in the ECU harness? Is it pin 3?

Posted by: stauf Sep 9 2006, 03:17 PM

So how many miles does it take the ecu to relearn the fuel maps...

I've heard different info on this subject for quite awhile now, so i'll take another opinion here.. =)

Posted by: ricerrx7 Sep 9 2006, 03:26 PM

DJ, seems that we lost the diagrams in the other tread, so that's something you'll have to look up on hmaservice.com.
Stauf, I believe that it takes 3 drive cycles for the effects of the reset to be lost.

Posted by: stauf Sep 9 2006, 03:29 PM

Another thing I've never gotten a full answer to.. Define Drive Cycle.

Posted by: cclngthr Sep 9 2006, 04:32 PM

Drive Cycle=Key on, drive for 3-5 miles (normal driving) at speed, making sure you up/downshift from 1-5 and back 5-1 (auto 1-4/4-1) then shut down (key off) then a wait period (I think 1-5 minutes).

Posted by: stauf Sep 9 2006, 09:27 PM

Colin to the rescue... Is that a set pattern the ecu has defined as a drive cycle, or are there variables included in the logic? (sorry, programmer in me comming out)

Posted by: ricerrx7 Sep 9 2006, 10:25 PM

I believe that the in order for the computer to see a full drive cycle, it has to be able to run through all of it's tests. And what colin described is the amount of time needed for this to happen.

Posted by: atlwteboy240 May 13 2007, 09:16 PM

i dont really understand the DIY for this what is the swich i didnt see anything tou rurn on or off mabey im jusy a dididi but please help me figure it out.

Posted by: RHINESEL May 13 2007, 10:14 PM

What do you need help with, understanding what the ECU is or with the DIY?

The ECU is what people think of as the car's "Computer". It takes a variety of imput signals and adjusts the engine to give the best programmed result. Our ECU's have a learning curve meaning that they also use historical information.

When you do certain modifications you change what these inputs are. For example, a CAI will change the airflow and temperature of the air from a stock airbox setup. Over time, the ECU will adapt to this. If you want to force the ECU to take the new inputs immediately you can "reset" it with a variety of methods.

Some like to reset it before racing events. Reseting it brings it back to factory like (some factory, some historical) settings that give slightly more power to the engine.

The three methods for reseting are as follows:

1. Pull the battery cables and step on the brake pedal.
2. Remove both ECU fuses
3. Add a switch via DIY to turn off the power to the ECU.

Any other questions feel free to ask.

I'm sure I made a mistake in here somewhere, anyone feel free to correct me!

Posted by: silet May 13 2007, 10:17 PM

I did

Posted by: RHINESEL May 13 2007, 10:26 PM

Where? Don't see it.

Posted by: Toecutter May 13 2007, 11:09 PM

Drive Cycle:

Posted by: atlwteboy240 May 14 2007, 05:35 PM

my question was about the DIY where is the swich in it i didnt see any swich all i saw was taping into fuses and the blue wire. how long do you take out the fuses for or do you just unplug both then put them back in?\

Posted by: ricerrx7 May 14 2007, 05:41 PM

It was assumed that if you can get that far, then you can figure out how to mount the two wires to a switch. Run the two new wires to where ever you want the switch, and connect them. But you only need to remove the fuses for a few seconds.

Posted by: Midnightsky May 14 2007, 08:12 PM

So in all reality all I have to do is make a break in the blue wire in that specific harness and place any kind of switch in the middle of the break? Makes sence to me, I am an Electronics Engineer Undergradute and as far as I've learned that is the correct way to setup a disruption to the power. Now......ill effects after doing the switch and resetting the ECM alot.......so what are we talking about.....like loss of fuel mileage? The ECM getting fried or just wearing down quickly? (how quickly?) anything i'm missing please fill in cause I'm seriously considering this .......my mods are an SRI w/K&N, deleted second cat, baffled res, dynomax muffler.......future (next 2-5 months) PnP IM w/Phenolic spacer, Fidanza flywheel, upgrade exhaust pipe from stock-2.5inch.

Posted by: ricerrx7 May 14 2007, 08:23 PM

You just completely compexified the simpleness of "wire in a switch". LOL
The one downside that has been mentioned is that is has the possibility of killing the cat. The ECM learns the fuel trim and leans out the fuel to accomodate for emissions. So everytime you reset the ECU it runs a little bit richer than what is intended. Running raw fuel through the cat can cause degredation and eventually kill it. But as long as you don't reset it everytime you start the car, I can't really see this becoming a problem.

Loss of fuel mileage? That's debatable, as some people have actually claimed better mileage after a reset. No harm to the ECU itself, unless you do a really bad job of wiring.

I've got the switch installed (obviously), and have had no problems with it. Granted, I've used it like 3 times in the last year...

Posted by: RHINESEL May 14 2007, 10:24 PM

1. Sorry, yeah like Ricer mentioned you can put the switch where ever you want to. Some drill a hole in a blank switch cover and put a switch there.

2. I just unplugged the fuses for a couple of seconds. Unplug, take a sip of water, scratch the rear, pick your nose or whatever. No big deal.

Posted by: thunderabeGLS Jul 1 2008, 09:46 AM

i reset my ECU yesterday.

read about it on http://www.elantraclub.com/forum/index.php?autocom=blog&blogid=11&showentry=66

Posted by: Elantra2000 Mar 12 2009, 03:25 PM

On mine I only found one ECU fuse underhood... Where's the other one?

Posted by: sed Mar 12 2009, 03:45 PM

behind your change bin

Posted by: SideoftheMoon May 31 2014, 06:11 PM

Change bin? The spot I throw my loose coins? I see no fuses anywhere near there. Please explain a little more.

Posted by: mlumz May 31 2014, 07:18 PM

To the left of the steering wheel near your knee is a bin usually used for change. Just pull the bin out and there are fuses behind it