ELM327P datasheet(22/51 Pages) ELM

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Resetting Trouble Codes

The

ELM327

quite

capable

resetting

diagnostic trouble codes, as this only requires issuing

a mode 04 command. The consequences should

always be considered before sending it, however, as

more than the MIL (or ‘Check Engine Light’) will be

reset. In fact, issuing a mode 04 will:

- reset the number of trouble codes

- erase any diagnostic trouble codes

- erase any stored freeze frame data

- erase the DTC that initiated the freeze frame

- erase all oxygen sensor test data

- erase mode 06 and 07 information

Clearing of all of this data is not unique to the

ELM327 – it occurs whenever any scan tool is used to

reset the codes. The biggest problem with losing this

data is that your vehicle may run poorly for a short

time, while it performs a recalibration.

To avoid inadvertently erasing stored information,

the SAE specifies that scan tools must verify that a

mode 04 is intended (“Are you sure?”) before actually

sending

the

vehicle,

all

trouble

code

information is immediately lost when the mode is sent.

Remember that the ELM327 does not monitor the

content of the messages, so it will not know to ask for

confirmation of the mode request – this would have to

be the duty of a software interface if one is written.

As stated, to actually erase diagnostic trouble

codes, one need only issue a mode 04 command. A

response of 44 from the vehicle indicates that the

mode request has been carried out, the information

erased, and the MIL turned off. Some vehicles may

require a special condition to occur (eg. the ignition on

but the engine not running) before they will respond to

a mode 04 command.

That is all there is to clearing trouble codes. Once

again, do not accidentally send the 04 code!

22 of 51

ELM327

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Quick Guide for Reading Trouble Codes

If you do not use your ELM327 for some time, this

entire data sheet may seem like quite a bit to review

when your ‘Check Engine’ light eventually comes on,

and you just want to know why. We offer this section

as a quick guide to the basics that you will need.

To get started, connect the ELM327 circuit to your

PC or PDA and communicate with it using a terminal

program such as HyperTerminal, ZTerm, ptelnet, or a

similar program. It should normally be set to either

9600 or 38400 baud, with 8 data bits, and no parity or

handshaking.

The chart at the right provides a quick procedure

on what to do next:

Ignition Key to ON,

but vehicle not running

>0101

to see how many codes

(2nd digit of the 3rd byte)

>03

to see the codes

(ignore the first byte and

read the others in pairs)

>04

to reset the codes

FIX THE VEHICLE!

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