How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask Robert Your Own Question

Robert, UK Qualified Car Technician
Category: Audi
Satisfied Customers: 4340
Experience:  Fully qualified 19+yrs exp/self employed technician in the UK
Type Your Audi Question Here...
Robert is online now

I am potentially buying a 2011 Audi A5 from a Trade

Resolved Question:

I am potentially buying a 2011 Audi A5 from a Trade Seller on AutoTrader, i have been to see the car and its partner test drove it and has some concerns regarding the power of the car.
He felt it really struggled to move off from stationary and when moving off from traffic lights etc. Its a 2ltr sports verison and should hold some ooomph....when we took this up with the seller he gave a long spiel about it needing a CO2 something replacement which they were already planning to do, he told me to Google CODE P0400 pcm. I did that...not much came up.
My question - Do you believe the CO2 story for the reason behind the lack of power? My partner thinks the Turbo has gone but we are not car wise, the seller was adament it is not the turbo.
Any guidence/info you could share would be invaluable and may save me a dodgy £20k spend!!
Thank you,
Submitted: 1 year ago.
Category: Audi
Expert:  Robert replied 1 year ago.

Was there any warning lights up on the dash when you test drove?

Any unusual excess smoke that you noticed out the back?

Whats its mileage / service history do you know?
Customer: replied 1 year ago.

Thanks for coming back to me.

There were no warning lights on the dash when we test drove but the seller said there HAD been one, which was why he was getting this CO2 part replaced.

I didnt notice any excess smoke coming from the car.

Mileage is 39k and has a full Audi service history...although i dont know why the previous owner didnt then sell to Audi and chose a small time garage instead...seems a bit odd to me! :)



Expert:  Robert replied 1 year ago.

Ok.. As i know with that code you mention its nothing to do with a CO2 or c02 sensor/lambda,oxygen sensor or what you want to call it..those sensors are on the exhaust system and monitor the flow of exhaust gases that pass through the system and thus the ecu can then alter mistures fuel,air wise - the code p0400 is an Exhaust Gas Recirculation (EGR) System: Malfunction or flow issue - exhaust gases flow through that or by it and all that egr does is takes a a volume of exhaust gas back into the engine and these valves often get coated or stuck open or shut with soot/carbon..when they do that then this can trigger a engine warning light and normally set that fault code you mention..where then its a simple case of either removing the egr valve and clean it up or replace it..which ever needs to be done based on testing..when those valves stick etc..they can cause a lack in power,engine idling issues or even starting if thats the fault code they found - then that would be the area of investigation..Its not a serious fault as most if not all cars on the road nowadays especially diesels suffer with these problems..and its more so down to general use of the vehicle and not allowing the engine to burn that carbon/soot deposits off when on longer use journeys......My advice.keep options open in regards ***** ***** better examples eleswhere..let the trader fix the issue's..once its been done then request another test drive and see what you think after that..if the same fault is present..then maybe look around elsewhere to be honest..because your going to spending a fair amount of money on the car then the trader should understand that ensuring a sale - then that car should be 100% in working condition..if possible maybe ask for independant inspection done on the car if the trader allows..i do know AA / RAC do such inspections aswell as - HPI checks on the car - unless you know of good local garage you use and get them to give it once over..before setting anything in stone buying it etc....
But as it stands but obviously i cant see the car for doesnt sound like possible turbo would know it had because you would have virtually no power / the turbo screaming/whistling loudly and depending on the nature as for failure leakage of engine oil around the turbo body or blue smoke out the back due to turbo seal failure...I do hope this helps..if you require to ask anything further then just continue before you rate
Robert, UK Qualified Car Technician
Category: Audi
Satisfied Customers: 4340
Experience: Fully qualified 19+yrs exp/self employed technician in the UK
Robert and other Audi Specialists are ready to help you
Customer: replied 1 year ago.

Thank you for such a detailed reply.

Just to clairfy, in your opinion, could the P0400 EGR issue potentially be causing the lack or power/struggle from the car?

I will still however get an independant inspecation on it before any funds are exchanged.

Thanks again,


Expert:  Robert replied 1 year ago.

It is possibe for a faulty or stuck EGR valve to cause power issues like you experiance so its certainly something to look into id say...or to discuss with the trader again what code it was and testing done etc to come to that conclusion in regards ***** ***** part they are replacing ..if thats the code they got when they scanned it of course and not just some code off the top of their heads.....

but hope all goes well ..if you have to ask anything further then just ask anytime..please dont forget to rate the answer above most appreciated.. regards Robert

What Customers are Saying:

  • 9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much. Phil
< Previous | Next >
  • 9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much. Phil
  • Lurch. Thank you very much. I had real doubts about this website but your promptness of response, quick followup and to the point answer with picture was incredible. Charles
  • As a single woman, I really appreciate an excellent and affordable opinion.
    Thank you Geordie, I will not hesitate to contact justanswer in the future!
  • Another great insight to what may be the problem. I will have my mechanic take a look at it tomorrow. Thanks again, do indeed know your stuff. Jim
  • I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry. Stephanie P
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C.
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex

Meet The Experts:

  • Matt


    Mechanical Engineer

    Satisfied Customers:

    BEng hons Mech engineering, in auto industry 22 years
< Last | Next >
  • Matt's Avatar


    Mechanical Engineer

    Satisfied Customers:

    BEng hons Mech engineering, in auto industry 22 years
  • AudiDoc's Avatar


    ASE Certified Technician

    Satisfied Customers:

    Independent European garage owner. Over 20 years experience
  • mike_a_mechanic's Avatar



    Satisfied Customers:

    17 years experience as a auto mechanic, foreign and domestic.5 years running my own repair business.
  • Robert's Avatar


    UK Qualified Car Technician

    Satisfied Customers:

    Fully qualified 19+yrs exp/self employed technician in the UK
  • Greg's Avatar



    Satisfied Customers:

    Automotive diagnostic tech
  • Ross's Avatar


    Diagnostic and repair technician

    Satisfied Customers:

    Licensed diagnostic and repair technician