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I have got a few issues with my POLO TDI 2012. My car starts and runs, one day when I start it in the morning, car idles roughly (rpm jumps up and down quickly). Then the car starts acting weird, like the head unit goes off for no reason, headlight dims sometime(especially when reversing). The main problem is I found there is a big time gap between changing gears, I assume it's a gearbox issues but the problem sometimes goes away, and when I go uphill the car refuse to change gear, I have to push on throttle more, but it goes to limp mode after changing gear. I took it to mechanic they can't find the problem. OBD sensor reads turbo underboost code.

Here is what they have checked:

  • Smoke tests there is no vacuum leak in the system
  • EGR checked not clogged
  • Fuel filter

I just found my battery indicator shows red, but I asked a few friends they all said those problems don't sound like a battery issue.

Can it be the battery? or what else it could be.

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  • It sounds like you may have more than one issue at the same time, which can make it complex to track down. What do you mean your battery indicator shows red, do you mean the generator light comes on? How consistent is the issue?
    – GdD
    Sep 5, 2022 at 7:52
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    Welcome to Motor Vehicle Maintenance & Repair! The best thing you can do is, if you know there's an issue, eliminate it before you beat your head against the wall. If the battery shows as being an issue, remedy it ... whether that means charging it from a charger or outright replacing it. Your battery could be low enough on power to allow it to crank and start, yet not be enough power for the electronics to be happy, which could lead to a lot of what you are describing. Sep 5, 2022 at 11:02
  • I've checked with the battery shop but no luck. Battery shows fine, don't know what else it could be.
    – Illumon
    Sep 8, 2022 at 3:11
  • What does "...like the head unit goes off..." mean? Oct 5, 2022 at 23:42

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Have your battery and charging system tested. If necessary, repair or replace any bad parts.

Your engine power problems are not related to your battery. Did you take your car to a mechanic who is experienced and equipped to work on VW TDIs? If so, they should have factory or aftermarket scan tools, such as VCDS. If not, you need a different mechanic.

Your problem might be a bad MAF (mass air flow) sensor. If it is sensing less air flow than is expected from engine speed and load, this can throw a turbo underboost code and put you in limp mode.

One way to test MAF function on a VW TDI is to disconnect the plug from the MAF. If the sensor is totally bad, engine performance will improve noticeably. (The engine will default to computed air flow if it has no sensor input.) This will also turn on the check engine light, but when you re-connect the plug and start the engine a few times, the light will go off.

To test the MAF for marginal performance that would require replacement, the mechanic can plug their VCDS into the OBD2 port and read "Meas. Blocks" Group 3 to compare MAF numbers, specified and actual. If specified and actual are not close to equal, the MAF is bad and should be replaced.

Cleaning the MAF on a VW TDI almost never helps.

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  • Yes they have VCDS, the scan didn't show anything other than turbo. But I don't think they did check MAF. I'll have a try. Thanks for the info!
    – Illumon
    Sep 6, 2022 at 3:30

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