I've recently had a no start condition happening/2012 A4 B8 2L

[DazzA4]

Pardon, I attempted to move this thread to the correct place but find I cannot.

Thanks in advance for any light that can be shed on the subject.

This occurs daily, especially when below 10 degrees (C) or colder, when attempting to start the vehicle first time.

The routine is: Remote key pressed in or in vehicle, dash lights come on, static battery voltage 12.6 (AGM battery of appropriate capacity, less than 6 months old, charges @ above 14V), foot on brake pedal, press start button to crank, dash lights go out, engine does not crank, CEL appears, and then at least 20 other codes appear, battery voltage does not drop below 12V.

So I scan codes, and always see a communication error between ECM/PCM "A" along with plenty of other codes (see list below). I clear the codes using the Carista app and Bluetooth dongle in the OBD port, often have to clear the codes two or three times, car then starts immediately with a strong cranking effort indicative of a good battery, idles smoothly during warm up, tho' sometimes the CEL stays on and often cruise control does not work until I do one more engine off clearing of codes. Car runs beautifully for the rest of the day, starts without fail, mileage is as expected, engine does not skip, CEL does not reappear until next day's start up routine. I do not have a VAG Com.

Subject: Audi A4 B8 2 Litre Turbo TSFI Quattro 8sp ZF Sedan, prod date 1/2012
Date: Thursday, 21 February 2019 07:13

OBD2 - confirmed:
U0100 - Lost Communication With ECM/PCM "A"

OBD2 - pending:
U0100 - Lost Communication With ECM/PCM "A"
U0100 - Lost Communication With ECM/PCM "A"

OBD2 - permanent:
U0100 - Lost Communication With ECM/PCM "A"

CAN network gateway:
01314 - Engine Control Module (ECM)

Ignition authorization:
01330 - Central Control Module for Central Convenience (J393)

Central electronics:
01314 - Engine Control Module (ECM)

Body/convenience system:
01314 - Engine Control Module (ECM)

Heater & air conditioning:
03220 - Stop Valve for Coolant for Climatronic (N422)
01314 - Engine Control Module (ECM)

Instrument cluster:
01314 - Engine Control Module (ECM)

Parking brake:
01314 - Engine Control Module (ECM)

ABS:
00473 - Control Module for Electric Park- / Hand Brake (J540)
01315 - Transmission Control Module (TCM)
01314 - Engine Control Module (ECM)

Transmission:
12551
12554
12562

Headlight aiming:
336388

Radio:
01314 - Engine Control Module (ECM)

[ELaw]

Two questions:

1. Does it seem like the engine tries to crank... is there a click or any noise from the starter?

2. When you say battery voltage does not drop below 12 volts, where/how are you measuring, and are you measuring continuously during the start attempt?

The reason I ask the above is that all those codes appearing at the same time sounds like a sudden loss of electrical power. And... I had a weirdly similar situation in a Saab I owned a few years ago that had an Optima AGM battery. When you turned the ignition on, everything seemed fine... all the electrics worked, and if you put a voltmeter on the battery it would read just over 12 volts. And I knew from experience the battery was in good shape and fully (or close) charged. But the battery's internal resistance had increased such that when you tried to start the engine, the additional current draw from the starter would cause the voltage to drop to zero. It basically acted like a bad battery cable connection, but the problem was inside the battery.

[DazzA4]

Using a digital multimetre attached to the hot post and earth post under the bonnet, I am watching the battery voltage as I press the "go" button in my attempt to start the vehicle. There is no audible click, the dash lights go out, and the voltage drops barely .2 or .3 volts. There is no crank, and no significant voltage drop befitting an attempt by the starter to turn. The battery, an H8-900CCA AGM, was load tested at the local Autozone yesterday and shows to be as new.

The cascading fault codes are, I believe, due to a failure to communicate; they all appear once the vehicle won't start. I am pulling the ECU and checking/replacing the '645' relays today, as well as checking the loom and multi-pin plugs in that area. I had a HPFP issue several months ago that was relay related so there might be some corrosion or moisture aggravating the situation.

[tech30six]

Using a digital multimetre attached to the hot post and earth post under the bonnet, I am watching the battery voltage as I press the "go" button in my attempt to start the vehicle. There is no audible click, the dash lights go out, and the voltage drops barely .2 or .3 volts. There is no crank, and no significant voltage drop befitting an attempt by the starter to turn. The battery, an H8-900CCA AGM, was load tested at the local Autozone yesterday and shows to be as new.

The cascading fault codes are, I believe, due to a failure to communicate; they all appear once the vehicle won't start. I am pulling the ECU and checking/replacing the '645' relays today, as well as checking the loom and multi-pin plugs in that area. I had a HPFP issue several months ago that was relay related so there might be some corrosion or moisture aggravating the situation.

Replace your ECM power supply relay. located in fuse panel under ECM. You'll be all set.

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[ELaw]

Hey I was just poking around the TSBs for our cars and found this: https://static.nhtsa.gov/odi/tsbs/20...55713-5611.pdf

It sounds an awful lot like the problem you're having. And as tech30six indicated, replacing the ECM power relay is the fix.

[DazzA4]

Thanks for posting the link to that TSB. It's good to have a copy downloaded now. I had seen it mentioned in my searches on the interwebtrons.

I haven't used the car much lately in all this strange snow and cold in the otherwise Pacific North Wet, so I've made no attempt to dive into the fuse panel beneath the ECU. I do have a spare 4H0951253A relay ready to go in case that is the drama. I continue to note that when I do have occasion to drive it and it's below 0c overnight, it always requires a code scan and reset, above 0c, not so much.

Success or failure report to follow.

[DazzA4]

It's a beaut sunny arvo after a soggy, snowy morning.

After digging in, replacing the 4H0951253A relay, and two dozen starts over several hours, the A4 now fires up first go each time, no codes, no CEL, no dramas.

Thanks heaps to all who offered up practical advice and useful information. It is a real money saver. I love getting into the nitty gritty and attempting to solve the problem with a bit of leg work and thorough research. I s'pose it's why most of us are here.

Cheers.



Next day edit: this morning it's a heavy frost and -2c. The car started first go. The cold seeming to cause more of a no-start issue was perhaps incidental.
The relay, Chinese-made, $8.49 at Autozone, bought two. OEM is currently $31.80+ in the US.

[fastboatster]

I got the same problem today, wonder if there's an autoparts store analog available. I don't want to wait to get it shipped or overpay at my local "friendly" dealer.

[DazzA4]

Old thread revival - but so often whilst searching online for information there's often never a follow-up done by the original posters so the future searcher doesn't know if it was successful.

So, to answer it for any who may care, I have not had a single recurrence of that cold start issue since replacing that relay, what, over five years ago. Bloody good.

[allstock]

Old thread revival - but so often whilst searching online for information there's often never a follow-up done by the original posters so the future searcher doesn't know if it was successful.

So, to answer it for any who may care, I have not had a single recurrence of that cold start issue since replacing that relay, what, over five years ago. Bloody good.

Good to see you figured it out. Been in those situations and it doesn't feel good. Cheers!

[Theiceman]

Old thread revival - but so often whilst searching online for information there's often never a follow-up done by the original posters so the future searcher doesn't know if it was successful.

So, to answer it for any who may care, I have not had a single recurrence of that cold start issue since replacing that relay, what, over five years ago. Bloody good.

thanks for the update .

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