Diagbox and obd reader are reading different values on same senor???

Help with sourcing, buying, installing and using Lexia, Diagbox and OBD2 for a Citroen C3.
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My Name: jmmj

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Model: Other
Year: 2014 (14)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 37089
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Gearbox: Automatic PRND
DPF: No
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Engine name: TU5 16-valve (110 PS)
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Hi folks,

I have a recent full chip 'diagbox' clone and a cheap obd2 bluetooth reader.

I have a pinging issue and wanted to look at the MAP sensor. Map readings should follow the TPS (throttle body position). On the obd reader it seems to be functioning fine as tested in two different apps. In diagbox, i have a correct readings at idle, but if i step on the accelerator the mbar value goes DOWN when pressure should be increasing?!?!

What does one do in this case??

Cheers,

Mike
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My Name: Arfur Dent

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Model: C3 2002-2005, Original shape model
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Gearbox: Manual 5 speed
DPF: No
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jmmj wrote: Fri Sep 10, 2021 6:38 pm Hi folks,

I have a recent full chip 'diagbox' clone and a cheap obd2 bluetooth reader.

I have a pinging issue and wanted to look at the MAP sensor. Map readings should follow the TPS (throttle body position). On the obd reader it seems to be functioning fine as tested in two different apps. In diagbox, i have a correct readings at idle, but if i step on the accelerator the mbar value goes DOWN when pressure should be increasing?!?!

What does one do in this case??

Cheers,

Mike

Hi

I would go with what Diagbox says because that is the dealer tool. You may need to update diagbox version if it doesn't work fully with your model.


As for cheap OBD readers, they are probably based on the same chip so carry the same error. I would take Diagbox/Lexia over OBD clones every time.
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My Name: jmmj

Contributor
Posts: 12
Joined: Sat Nov 21, 2020 6:09 am
Model: Other
Year: 2014 (14)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 37089
Trim Level: Exclusive
Gearbox: Automatic PRND
DPF: No
LHD or RHD: LHD
Engine name: TU5 16-valve (110 PS)
Has thanked: 11 times
Been thanked: 1 time

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Cheers Arfur!

Its strange, that was in the custom selected parameters pane. While in the standard parameters section the result was different. Well, i decided to hold the pedal down for a few seconds instead of just blips. So the pressure reading did actually go up. I think its a refresh rate thing, diagbox just seems really, really slow. Maybe its the computer, even though its dedicated to diagbox it is old?!!
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My Name: Arfur Dent

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Posts: 3340
Joined: Sat Jul 23, 2011 3:47 pm
Model: C3 2002-2005, Original shape model
Year: 2002 (52)
Engine Size: 1.4 (16v)
Fuel Type: Diesel
Mileage: 100000
Trim Level: Exclusive
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: DV4 16-valve diesel (90 PS)
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I don't know which laptop you are running on. But lexia can run on some very old hardware. Diagbox needs a bit newer for Windows 7.

A faster laptop may be better?

Maybe it won't make any difference?
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My Name: Ozvtr

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Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
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Simple OBDII readers don't have an operating system and I/O issues to deal with. And laptops are ssssooooo slow.

You are better off logging the data. Then studying (comparing) the traces. Like, MAP Vs TPS. It's a lot slower but you can get more information out of it.

Yes, a cheap 15 pound OBDII is very good for spotting basic engine problems. It's quick and you don't need to drag a laptop around. But when it comes to scanning and trouble shooting the other ECUs, LEXIA represents better value for money. While I have not used it, apparently there is an EOBD scanner in one of the versions of Diagbox. Arfur might know a bit more about that.
jmmj wrote: Fri Sep 10, 2021 6:38 pm I have a pinging issue
Have you logged engine load vs timing advance? As the load gets greater the timing should retard. The engine ECU will advance the timing until the knock sensor complains. If the knock sensor is un-serviceable, the ecu will keep advancing the timing. Not sure if there is an option to log knock sensor 'hits'. Knock sensors are difficult to test.
Could be crap fuel? Even at max retard bad fuel can ping.
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