Thank you all for your replies
According to the workshop manual and this thread here:
Tire pressure re-set tool, the horn should chirp twice (not once) at the start of the reset.
What my memory says is that it starts with a single chirp, and that the double chirp is a mistake in the manual.
I'm trying to diagnose why the new sensors are not being recognized and the difference between the workshop manual and my memory (and your reply I suspect) eliminates the first problem, however I would like confirmation that it is the manual that is wrong.
To answer your second question, I'm using a magnet, same as I have always done. However, even with the magnet clinging to the sensor it isn't being recognized. Getting the 'official' tool is something I forgot to do when I bought the replacement sensors. However, if there is
supposed to be a double-chirp at the start then I need to fix that problem first.
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As a reminder here is the reset procedure, as per the workshop manual, I have put what I think is the wrong text in
bold:
Select the ACC position on the ignition switch.
Simultaneously press the keyless entry transmitter lock and unlock buttons until a
double horn chirp sounds, indicating the TPM learn mode has been enabled.
NOTE
Important
If a horn chirp does not sound after 15 seconds using either tool, repeat step 1.
Starting with the left front tire, hold the antenna of the TPM tool against the tire sidewall close to the wheel rim at the valve stem location then press and release the Activate button and wait until a horn chirp sounds.
After the horn chirp sounds, proceed as in step 3 for the next 3 sensors in the following order:
Right front
Right rear
Left rear
After the left rear sensor is learned, a double horn chirp will sound indicating all sensor IDs have been learned.
Select the OFF position on the ignition switch to exit the TPM learn mode.