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2000 V6 Tiptonic FWD, P0605 & p1850, No start condition

1072 Views 4 Replies 2 Participants Last post by  Tomvw
Hello, Not sure if this should be in garage or general section but here goes.

Picked up a 2000 GLX v6 FWD at an auction 67k miles, Clean, Timing belt says it was replaced just 1500 miles back based on sticker in engine compartment.
Found some of the last service records with old owner contact info, sent email, original owner was last owners mom, she passed, car sat for little while, her son drove it for a year, donated to charity.. beyond that she knew nothing.

Got 2 sets of keys (with working fobs), no valet key and no key code tag.

The car would jump at auction and start, I knew the battery was completely shot, the flex pipe is busted on the exhaust.. I thought this was the extent of the car's problems as it seemed clean and maintained otherwise, check engine light was on when it was running but it also complains about emissions so I just assumed that was the CEL, Starter cranks smoothly, no drag, no grind.. Engine fluid levels are normal and clean, oil change is almost due via time but milage only about 1,800 since last. Engine sound smooth (ear to it while running)

So I win it, and get it home which was only a mile from auction, get a new battery in it, starts up.. turn it off.. won't start back up.

I do not have a vag com cable, I do have a ELM code reader.. it shows codes p0605 which is a rom error.. I assume ECU?
The other code is p1850 power train databus error.. I assume this means it can't communicate with the TCM.. although from searching here perhaps the p0605 could be TCM error?

OBDII comm with the ECU however works fine.. no issues connecting and reading.

I checked all the fuses that were marked with an engine on the fuse card.. all were good, everything else in the car appears to work, windows, keys, mirrors, radio, etc..

I disconnected the battery, reconnected it 30mins later, car started right up. no problem.. after about 5 mins the car went into Limp mode.. I actually did not even know what limp mode was at the time, The car felt like it had slow acceleration but after about 45mph the car felt like it drove fine. (have to remember I never drove this car before I bought it so I have nothing to compare it to)

Drove it for 60 miles (probably not the greatest idea now that I know what limp mode is) got home, turned it off. you guess it.. won't start again.
I've gotten the car started a few times this way.. so it's repeatable by yanking the battery for 15-20mins.. sometimes I get emission shop warning.. sometimes not but I figure that's exhaust related which I will have fixed once the car is reliably running, I don't imagine a busted flex pipe in the exhaust is causing all this.

Im operating on the assumption the ECU is bad, so I opened the box and removed it.. I noticed one of the screws was missing, This makes me wonder if the son who had it might have tried to monkey with it or if perhaps someone prior had looked into the ECU.

Expecting to find lots of water but the lid was tight, seal seems good, I see zero signs there was ever any water in the box.
I looked down the hole and did not see any obvious problems with the wiring, I gave them a tug to make sure the harness was connected.. it seems so.
The ecu looks pristine, connectors show no signs of corrosion, I reconnected the ecu but did nothing to improve the situation.

I checked the fuse in the ECU box, it's good, I checked the relay in the box a bosch 5pin, it tested good as well.
I cracked open the ECU shell, no signs of water, corrosion, bulged caps, cracked or blackened components.

Electronics don't always show outward signs of problems but nothing was obvious.
ECU P/N is: 3B0 907 551 AN

I was lucky and found one (seller swears good pull although looks a bit rough) for 12 bucks so I went ahead an ordered it
It may not be the ECU, but for 12 bucks for what can be a very costly and difficult to source part, not much to loose.

So advise on what I should do next? My family has several mechanics but none of them know much about VW's and no one I know owns one.
Basically the car is so clean and pristine for it's age it seduced me at the auction.. probably bit off more than I can chew but im convinced mechanically she is sound.

So here's a few questions

#1 Can I simply plug'n'play the ECU if it's an exact part match?
I tried searching this and got the phone book

PZ over on this thread https://www.passatworld.com/forums/b5-garage/327168-ecu-swap-help-md-dc-nova.html
Says 2000 should be plug'n'play, True?

.. I would have just assumed some things needed programed/changed

.. What about Throttle body alignment? I've heard this can be lost when battery is changed, mine seems to not care but does that need redone with a ECU swap?

.. I don't think my car has an immobilizer.. I haven't seen it on the cluster panel, and I read that if you a W with a line thru it on your key your car does not have immo, and a few threads over here suggested none of the NA 2000's had it, but over here at ross tech if I understand it correct 2000's should have Immo 2.. so Im a bit confused on this
Immobilizer - Ross-Tech Wiki

#2 If I swap the ECU will that cause any of the other module to get made and have faults (not seeing original ECU)
.. basically is it safe to at least try a ECU swap?

#3 I've become aware of the water in the common floor board situation with these cars, regardless of the problem currently Im planing on checking the CCM and TCM under the carpet.. but I could not find a youtube video showing how to get up the carpet.. do I need to remove the seats? or can I just take trim up and pull the carpet back without having to unbolt seats?




Sorry if the post is a bit long but I wanted to give as much background info as possible.. please advise I'd really love to get this car on the road it's gonna become my daily driver.
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If the replacement ECU is from a car with tiptronic it should be plug and play. If from a manual it will need re-coding.
The P0605 code indicates a faulty ECU, the P1850 would be caused by the faulty ECU.
After replacing the ECU, switch the ignition to the on position (Do NOT start) and leave on for 3 minutes, the throttle body should adapt. Listen for whirring noises from the throttle body.

You don't need to remove the seats to get to the CCM and TCM, but I suggest you don't touch them unless you get related codes.
Thanks for the reply, I just wanna pull the carpet up and check for water.

As for the ECU how can I tell it came from a tiptonic vehicle? the part seems like a perfect match minus what I think is a serial number it looks the same.
Only number different is highlighted in red which I took as just a serial number.

Original: http://i64.tinypic.com/2uzzd02.jpg
Replacement: https://i.ebayimg.com/images/g/C48AAOSwsZhcJVu7/s-l1600.jpg

EDIT: fixed image links, can't seem to get both to show in thread.
TL;DR: Car starts on command with replacement ecu, p1850 code is gone but p0605 persists, no dash lights.. still entering limp mode / CEL


Well ECU finally came today, 2 days late.. oh and post man for some reason didn't deliver it till 6:30pm.. thanks mate :(

I put it in the car and did like you said to turn it to on but not crank, horn and lights start blaring with the EPC light on.

I did not hear any mechanical noise like the the throttle body was being interrogated.

after about 10 mins of messing with it and the horn/lights kept going off I yanked the cable to clear it.
hooked it back up and tried again.. this time no lights and horn left it "on" for 5mins.. still no mechanical noise, tried to start it.. started right up.

check engine light was on check it with the obdII reader.. and it said p0605.. but at least the data bus error was gone (p1850)

I cleared the code, let it idle for about 10 mins, I didn't have any dash lights unless I manually turned on the head lights.
Radio was saying "safe" I hadn't had this problem before on the old ecu disconnecting the battery.. luckily I have the manual with the code.
re-synced the remotes (a point for VW for not requiring "dealership" tools for that).. but nothing seem to bring the dash lights on.. I don't know if this is somehow disabled in the ECU? I know you can set features with a vag cable.

While it was idle it went into limp mode.. I drove it around the block once, parked it, turned it off, figuring it wouldn't start back up.. but it did.
I drove it around the neighborhood for about 1 1/2 miles or aprox 5mins of drive time..

emmissions shop warning came on, went back into limp mode, and the engine light came on.. pretty much all at the same time.
As I said the exhaust has a break in the flex pipe so the emmissions shop warning is no shocker but that wouldn't cause the car to go into limp mode would it?

I parked back at home turned it off, .. then started it up.. started up again no problem so we've made some improvement here.

It does not start in limp mode.. that happens after a mile or so or about 5mins.
So when I started it this time I decided to try the tiptronic feature which worked with out issue and was a lot of fun.. although the 1-2 gears are too short for my taste, you hit them almost immediately, and boy does she have pep.. a lot more than I expected honestly.

That's all well and good until limp mode hit :/


So im sitting here thinking.. where do I go from here? order a 2nd replacement ECU?
I mean it's possible the guy sent me a bad ecu.. but im kinda doubting that.. what has me worried perhaps there something damaging the ECU?
Is it possible I put this replacement in and it fried part of it from some other problem?

At least the car starts when you want it now.. but im getting frustrated.

P.S replacement ecu had 2001 passat written on it in marker, I guess that doesn't make any difference though, p/n's matched right?
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"Car starts on command with replacement ecu, p1850 code is gone but p0605 persists, no dash lights.. still entering limp mode / CEL"
Are you sure you didn't list these codes the wrong way ? The P0605 indicates a faulty ECU.

If the ECU is from a manual, this would cause transmission limp mode.
If the ECU is from a manual, you can re-code it for Tiptronic with VCDS.
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