Replacing original ECM with used ECM 2008 Audi a4 3.2L


First of all, I’ve added this to my previous CEL problem back in Dec '19 and Jan '20

Original ECM being replaced by a used one but both have the exact same Audi part number and manufacturer’s number. My Audi is a 2008 A4 3.2L Quattro Avant with Tiptonic auto transmission.

Original has had a P2404 (EVAP problem) for some 4 years that the dealer has declared the ECM is at fault and needs replacing. All tests including wiring and vacuum lines are ok.

They want $2,439 + $80 to set it!

In addition they said the vacuum was 19.1 inches at the leak detection pump and should be above 20 so they want to replaced the booster vacuum pump for $858!. I bought a new one from a supplier in Phoenix, AZ for $48.56 + a gasket locally and replaced it myself. Not easy but worked out fine with vacuum now at 20.5

I bought the used one from a parts salvage yard in Lithwania, $123.xx including shipping, which was delivered via DHL in three days two hours! (pretty amazing!) I replaced the original with the used one without disconnecting the battery (bad me but…) then turned on the ignition without starting. SAFE appeared where normally the total mileage would. The trip odometer remained at 157.X. OBD II meter had 6 codes: 4 for the cam, 1 for fuel pump, and the dreaded P2404! The VIN number was different but checking on it showed the ECM came from a 2008 Audi sedan with a 3.2L engine.

Question: should I clear the codes and start the engine? Anyone’s suggestion here would be greatly appreciated.

I’ve posted here before about fuses I could not find for the ECM but after removing the improperly secured case for it, discovered the fuses and the TOTALLY corroded and rusted relay switches that causes some 9 code errors just a few months ago. Both replaced and I’ll put that episode in this forum sometime latter with photos.

Here’s a question for YOU . . . can you even start the engine with this used ECM?

I don’t mean to sound like a smart alec, but those modules usually are married . . . or marry themselves after x number of drive cycles . . . to the vehicle, meaning a diy guy can’t get them to work on another vehicle. There are businesses that are able to do this, but that typically goes way beyond what you can do in your driveway or garage

here’s what I would do first . . . see if the engine will even start

If it doesn’t, you’ve already got bigger problems

If it starts, you can always clear the codes later

I certainly hope ALL the tests were performed, prior to condemning the module. It has been my experience that far more than 1/2 of them get needlessly replaced, due to incompetence and/or incomplete diagnosis


The dealer’s Master Audi Mechanic did appear to do all the tests as he’d made a video of the LDP (located aft of the LR wheel and splash guard that had been removed) with its vacuum line disconnected saying that the vacuum was low, thus the need to replace the assist pump, which he also made in the video saying it was leaking oil. In the text from the dealership prior to me leaving the facility the mechanic stated “ran gff found faults P2404 & P2402; EVAP system …” along with other faults my OBD II meter did not detect but related to electrical sensor for electro-hydraulic engine mount. “all wiring to the LDP was checked from ECM ok at less then 1.0 ohms end to end and the LDP was checked electrically as well - sensor resistance is 680 (spec is 640-720) and solenoid resistance is 15 (spec is 13-28) both ok. voltage supply is battery voltage- ok. mechanically tested LDP and found it to operate as it should. ECM and vacuum pump condemned.”

He also in person and with the dealership’s customer service person told me that this problem, P2404, was a common problem with the 3.2L engines, which as I noted when I looked into the codes of the “new” used ECM appeared to be as he said. If this is in fact true then why has Audi not flagged it as a needed recall? In my state that code triggers a will-not-pass-emissions. My particular car has been recalled twice, each time for the passenger air bag; once for a possible canister metal explosion sending out shrapnel and the other for the entire thing to be replaced, both of which I had them do.

I am aware through following numerous conversations by Audi mechanics in the AudiWorld forum that VIN and mileage content can be set on replacement ECMs. They mention pin #81 most of the times. My posting on AudiWorld bring up no comments; only here in CarTalk.

So I am hesitant in starting the engine yet–being somewhat fearful that the replacement ECM may cause one or more problems thus my also wondering about the timing of reseting the fault codes already being shown for it. Start the engine as you suggest and if ok, then reset codes. It’s like as a teenager standing on a bridge 60 feet above a river and jumping for the first time even though there are swimmers below who will help if anything goes wrong! That was my predicament at age 14 or 15. Sh_t or get off the pot!

I’ll wait another day or two hoping for other comments and I absolutely appreciate yours.

What about calling the local Audi dealer and ask if it can be towed there and they set things properly ?

They also suggested that the day the diagnostics was done. I had removed the cover of the ECM compartment to save some mechanic’s time (and costs to me-I live on SS), which I had noted earlier had never been replaced properly by a previous mechanic from a CA dealership and which allowed water to enter and screw up the relay switches. So I’m gun-shy of dealers with magnificent buildings! My car runs fine with the original ECM, no need for towing. I am seeking further suggestions on starting the engine with the used ECM.

Other than agreeing with db4690’s comment about many ECMs being diagnosed and replaced needlessly because of a misdiagnosis I might add something here. That is in relation to the vacuum reading and apparently the vacuum pump was diagnosed as the source of the claimed lower than normal vacuum. I also fully agree with db4690 about trying to start the car. He’s pretty sharp and I don’t think he will steer you wrong.

There are a number of things which can cause a vacuum variance; and especially in a case with such a slight variation.
Altitude, engine wear, barometric pressure, rain or humidity in the air, ambient air temperature, variations in ignition timing from the ECM based on inputs regarding air temps, engine temps, etc, etc.

Point being is that maybe that increase from 19.1" of mercury up to 20.5 was not so much the pump but one or more of the factors mentioned. I don’t know where you live or climate stats on the days in question but maybe this will help.

But how are “they” erasing the previous VIN and inserting the correct one?

Factory scan tools . . . unmodified, as delivered to the dealerships . . . are not capable of clearing an existing VIN and mileage. They can only enter mileage and VIN on a “clean slate” module, meaning one which hasn’t yet been married to a particular vehicle.

I suspect these audi guys may be literally doing this on the bench . . . meaning the module is on the workbench . . . and they’re probably using some device which you either legally can’t buy, can’t afford, or aren’t even able to find in the first place. This is not like getting a breadboard at Radio Shack and soldering in resistors and LEDs

What about the mileage?

And how does pin #81 come into play?

Did you choose a username and join the forum?

Did you start a new discussion?

I’m asking this, because it has to be crystal clear not only that there’s a new guy, but that he has a very specific question. And you need to ask how this #81 pin comes into play. The only thing I can think of is that they’re clearing the existing VIN . . . because I mentioned earlier that the module will eventually marry itself to the vehicle. So they may be forcing a divorce, so to speak. But I’m not sure how they would/could be so selective, as to clear the VIN, but nothing else

Call me a pessimist if you want, but I think there’s a very good chance the car won’t start on that used module as is . . . unless you can get it to learn your VIN instead of the previous one

I believe you can’t do any harm trying to start. You’re not going to fry other modules or anything like that. The most damage can be done by static electricity, if you’re not careful . . . that can definitely fry a module. But since you’re not messing around with any of the other modules, there’s no danger of that, correct? You’re not messing around with other control modules, are you . . . ?!

Either start will be authorized . . . or it won’t

How are you going about all this, anyways? . . . do you even have a scanner hooked up?

If so, are you able to navigate to the screen that says if the start is authorized or not?

If so, what kind of scanner are you using?

Ok, I am at 5100 feet, Fort Collins, CO. Mileage is 172,6XX. Pin #81 came up for the VIN as I read the comments in AudiWorld.

I had registered with AudiWorld and posted my problem. No answers. I have noted the pin numbers inside the male connectors of the ECMs. I’d read in AudiWorld of one guy who replaced every item, one at a time so’s to keep track, within the EVAP system but still the code came up. He then bought and installed a used ECM that his dealer re-programmed but the car would not run properly so he put his original back in but before doing that he cleaned all the contacts, of which there are a lot! The car ran well and the code, after being cleared, did not return. I also cleaned all the contacts but didn’t help, thus the purchase of the used one from Europe. By the way here’s another funny. With the original ECM and original vacuum booster all other code sensors would be ok, that is a check with a green arrow or the like but not the EVAP system, which took a lot of miles and on/off drives. With the new vacuum pump, the P2404 would show up within 5 miles of driving to and from a grocery store: shut down, buy groceries and go home in which during that drive the CEL came on! I cleared it the next day and the same thing happened. I also discovered through AudiWorld forum that a pump comes on in the LDP EVAP system some 5-8 hours after the car shuts down, like during the night. That one got me also.

Tomorrow, kind of tired right now, I’ll give a try with the used ECM to start the car and see what happens. And no, I’ve not gone into any other modules. Curious, as my eldest son who works on his Bonneville Flats race car and his son’s super modified Lexus asked me last night “Does the trip mileage stay the same?” I said it does and both he and his other son were somewhat taken aback at that. My scan tool is an OBDII/EOBD by CEN-TECH.

Really, I do very much appreciate all of your suggestions.

Ok, here’s the skinny. Five minutes ago I exchanged the original with the used ECM; attempted to start engine, which once you turn the key to start it is no longer required to hold the key in this position as the starter remains engaged. Several seconds: no start. Key off. Second attempt: no start. Key off Third attempt: no start.

I’d connected the OBD meter and codes that appeared yesterday continue the same. Checked VIN number: same-- not my car.

Ignition totally off, key removed. I added my trickle battery charger that is fully automatic just to make sure there’s full voltage going to the car. I’ll wait most of the day and try again to start.

Intermission, so to speak.

@db4690 I may have overstated mechanics changing the VIN numbers but here’s a site that very specifically presents devices for changing mileage: . And within that site a link for the exact steps to do so: .

Attempted another start without success. Cleared the codes and the starter would engage only if key held in start position. Replaced used ECM with original and all’s back as previous, which includes P2404. Will look into having the used ECM reprogrammed or flashed.

There is an immobilizer system mismatch.

There is an ID code from the immobilizer module registered in the PCM. This is done so a thief can’t bring his own key and immobilizer module and start the engine, the PCM won’t recognize the replacement module.

I don’t know if the immobilizer ID code can be rewritten in the PCM.

I’m going to be brutally honest here . . .

I suspect the “bad ECM” diagnosis is no good

As I’ve stated many times, I believe over half of the times that an engine control module is replaced, it’s because whoever was diagnosing the problem(s) skipped a step, misinterpreted the results, or isn’t nearly as smart as they think, etc. . . . end result the same = module gets needless replaced, costing the customer more money

In plain english . . . I think even when/if you get that used module configured for your car, the P2404 problem will remain

And it’ll remain, until it’s properly diagnosed and repaired

And after it’s repaired, somebody will have to “verify the repair” . . . which takes brains and patience

@Nevada_545 @db4690 I’m indebted to you for your knowledge. I will continue on these two levels: 1) immobilizer ID code, which will be via the Net with probably no real good answer and 2) continued looking for the physical problem as db4690 recommends. Maybe a dealer with a smaller obsession with income?

Hoping not to have any gendarmes looking too closely at 3 months beyond renewed tags. Emissions passing remains staunch in this county of Colorado.

Thanks, mucho!

Here are the criteria, in my opinion, as to who is up to diagnosing it correctly

A middle-aged guy, because he’s got decades of experience

Somebody who is good at diagnosing electrical, drivability and/or emissions problems, and actually enjoys doing so

Somebody who has patience . . . a lot of it

Somebody who is very good at following trouble trees, flow charts, or whatever you want to call them

Somebody who is very good at looking at, and perhaps more importantly, understanding wiring diagrams

Somebody who can convert raw data, such as psi, inches of vacuum, and so forth, into electrical numbers, such as voltage, current, etc. Because that’s what it sometimes takes. Control modules don’t think in terms of vacuum, for example. They interpret signals from, say, a sensor. And it might be certain voltages that the module is looking for. If a certain code sets when x amount of vacuum isn’t achieved or maintained, you need to understand what the control module is actually looking for.

Somebody who understands and practices the concept “verify the repair”

That obviously rules out some 20-year old kid at a dealership, who probably just “graduated” from UTI or some such for-profit school

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Since the P2404 fault is present with the replacement PCM it seems that the problem is still present. You will need to check the leak detection pump circuits.


Actually I feel that since the car would not start there’d be no actions that would set the P2404 into the used ECM. That code appeared on my scanner prior to a start attempt.

Or perhaps that code and others are stored in a PCM separate from the ECM within the electronics of the car?

@db4690 I concur completely! So, for this middle-aged guy’s name and number within 500 miles of me, I offer a 3/4 full bag of salt ad pepper Pistachios that I received last Friday for my 81 birthday!