ELM329
Resetting Trouble Codes
The ELM329 is quite capable of 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 (among other
things):
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 it to the vehicle, as all trouble code
information is immediately lost when the mode is sent.
Remember that the ELM329 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 must not be running) before they will
respond to a mode 04 command.
- 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
- clear the status of the system monitoring tests
- delete on-board test results
- but will not erase permanent (mode 0A) trouble
codes (these are reset by the ECU only)
Clearing of all of this data is not unique to the
ELM329 – it occurs whenever any scan tool is used to
reset the codes. The biggest problem with losing this
That is all there is to clearing trouble codes. Once
again, do not accidentally send the 04 code!
Quick Guide for Reading Trouble Codes
If you do not use your ELM329 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 ELM329 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.
Ignition Key to ON,
but vehicle not running
>0101
to see how many codes
(2nd digit of the 3rd byte)
The chart at the right provides a quick procedure
on what to do next:
>03
to see the codes
(43 + # codes +
the codes in pairs)
FIX THE VEHICLE !
>04
to reset the codes
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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