Author Topic: o2 sensor testing?  (Read 318 times)

Offline hussnainh8

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o2 sensor testing?
« on: March 26, 2020, 03:33:52 PM »
Hi,

I have a GTI MK5 with very rough idle and cutting out issues. With the MAF disconnected the car runs fine and with the MAF connected the revs go up and down to where it eventually cuts out. I have replaced a number of parts but made no difference.  How can I go about testing the o2 sensor as my lambda control bank 1 sensor 1 (actual) is in -27.0%? I have no idea what this means. There are no engine codes showing other than I did ONES get P1297 which when I erased again, it didnt come back.

I have replaced the following:

Intake Manifold
Diverter Valve
Checked the cam follower in the HPFP
Checked boost hoses/vacuum hoses with soapy water. No bubbles.

Is there any way to test the o2 sensor if its working correctly.

VCDS readiness is showing oxygen sensor failed/incomplete. Is my sensor bad or do I have a vacuum leak somewhere that I need to do a smoke test to find?

Offline titchy

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Re: o2 sensor testing?
« Reply #1 on: March 26, 2020, 03:56:41 PM »
I would be looking at vac leak I had one on myMk5 it was  a coolant vac unit o/s when facing engine under vac pump £18 part genuine

Offline hussnainh8

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Re: o2 sensor testing?
« Reply #2 on: March 26, 2020, 06:12:59 PM »


Thinking about building one of these to test for leaks.. Whete would i put the hose too ? Into the air filter box or??

Thanks.

Offline muff1991

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Re: o2 sensor testing?
« Reply #3 on: March 26, 2020, 08:53:17 PM »
If your car runs fine with the MAF disconnected and rough with it connected would indicate a bad MAF!

The bad MAF could be causing the O2 issues. I’d start there.


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Offline hussnainh8

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Re: o2 sensor testing?
« Reply #4 on: March 27, 2020, 12:48:26 AM »
If your car runs fine with the MAF disconnected and rough with it connected would indicate a bad MAF!

The bad MAF could be causing the O2 issues. I’d start there.


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Sorry my mistake I didnt write it in my first post, the first thing I did was try another MAF sensor from my brothers car. Still same problem even with a new MAF.

Offline hussnainh8

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Re: o2 sensor testing?
« Reply #5 on: March 27, 2020, 03:20:28 PM »
New engine code!

1 Fault Found:
004759 - Pressure Drop between Turbo and Throttle Valve (check D.V.!)
P1297 - 001 -
Freeze Frame:
Fault Status: 01100001
Fault Priority: 2

On a cold engine, the car does its jumping revs to 2000rpm and down to 400rpm and cuts itself out, on a hot engine it runs and you can even drive it. The engine code showing is this. If you try to erase the code, it will erase and the revs will go crazy again and car will cut itself out again, when this code is showing, the engine runs but if you erase the code the problem starts again.

Have replaced the diverter valve yestarday which made no difference. Have checked for air leaks but literally cant hear anything.. Does it need smoke testing? or is the N75 boost pressure control valve faulty? Have checked boost pressures and the actual stays at 1010 and specified is around 460 when idling. When you rev the car, the actual stays roughly the same and the specified goes up to over 1000... Could it either be a air leak or the n75 valve needs replacing? Any way to check if the n75 valve works using VCDS?

Thanks.

Offline muff1991

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Re: o2 sensor testing?
« Reply #6 on: Yesterday at 11:23:06 AM »
New engine code!

1 Fault Found:
004759 - Pressure Drop between Turbo and Throttle Valve (check D.V.!)
P1297 - 001 -
Freeze Frame:
Fault Status: 01100001
Fault Priority: 2

On a cold engine, the car does its jumping revs to 2000rpm and down to 400rpm and cuts itself out, on a hot engine it runs and you can even drive it. The engine code showing is this. If you try to erase the code, it will erase and the revs will go crazy again and car will cut itself out again, when this code is showing, the engine runs but if you erase the code the problem starts again.

Have replaced the diverter valve yestarday which made no difference. Have checked for air leaks but literally cant hear anything.. Does it need smoke testing? or is the N75 boost pressure control valve faulty? Have checked boost pressures and the actual stays at 1010 and specified is around 460 when idling. When you rev the car, the actual stays roughly the same and the specified goes up to over 1000... Could it either be a air leak or the n75 valve needs replacing? Any way to check if the n75 valve works using VCDS?

Thanks.
your boost pressures actual will always rear around 1bar as thats the atmospheric pressure before boost. you are also not creating any boost on idle so dont worry about that figure. those figures should be almost aligned when the car is driving. A power run in 4th gear from 1.5k up until redline.. those numbers should then be close ish together to know everything that side of things are running. if your worried about air leaks and/or boost leaks.. you could look at the PCV valve?

if the O2 sensor is faulty this will also be causing issues.. or it could be doing its job and picking up on a fault.

if the ECU has picked up pressure loss between the turbo and throttle.. there must be a bad leak or faulty sensor in between! have you done a boost leak test? You could have a bad N75 or maybe a bad MAP sensor. TBH though.. probably a leak and thats why the O2 is picking up.