There is only one knock sensor. It's on the engine block, just below the starter.
Here's the info from the Mopar parts server:
SENSOR, Knock
#56028 563AA
It will be a warranty repair.
Diagnosis with a scanner will tell if it's working. Light tapping on the block will retard the timing. A scan tool will show the datastream.
From the 2003 PT service manual, page 8I-11:
KNOCK SENSOR
DESCRIPTION
The knock sensor is bolted to the cylinder block.
The knock sensor is designed to detect engine vibration
that is caused by detonation or preignition.
OPERATION
When the knock sensor detects a knock in one of
the cylinders, it sends an input signal to the PCM. In
response, the PCM retards ignition timing for all cylinders
by a scheduled amount.
Knock sensors contain a piezoelectric material
which constantly vibrates and sends an input voltage
(signal) to the PCM while the engine operates. As the
intensity of the crystals vibration increases, the
knock sensor output voltage also increases.
The voltage signal produced by the knock sensor
increases with the amplitude of vibration. The PCM
receives as an input the knock sensor voltage signal.
If the signal rises above a predetermined level, the
PCM will store that value in memory and retard
ignition timing to reduce engine knock. If the knock
sensor voltage exceeds a preset value, the PCM
retards ignition timing for all cylinders. It is not a
selective cylinder retard.
The PCM ignores knock sensor input during engine
idle conditions. Once the engine speed exceeds a
specified value, knock retard is allowed.
Knock retard uses its own short term and long
term memory program.
Long term memory stores previous detonation
information in its battery-backed RAM. The maximum
authority that long term memory has over timing
retard can be calibrated.
Short term memory is allowed to retard timing up
to a preset amount under all operating conditions (as
long as rpm is above the minimum rpm) except WOT.
The PCM, using short term memory, can respond
quickly to retard timing when engine knock is
detected. Short term memory is lost any time the
ignition key is turned off.
NOTE: Over or under tightening affects knock sensor
performance, possibly causing improper spark
control.
INSTALLATION - 2.0/2.4L
The knock sensor bolts into the side of the cylinder
block in front of the starter under the intake manifold.
(1) Install knock sensor. Tighten knock sensor bolt
to 22 N·m (195 in. lbs.) torque. Over or under
tightening effects knock sensor performance,
possibly causing improper spark control.
(2) Attach electrical connector to knock sensor.
(3) Connect the negative battery cable.
I've been on the Miata forum since 1999. Mine is a black 94 with the Jackson Racing supercharger, big boost kit, and NOS.
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