MACS Monthly Newsletter - 2017 - MAR4

If the sensor is shorted, the module would likely
set a DTC that has "OAT / AAT sensor voltage low"
in the code description. Conversely, when the sensor / related circuitry is open the primary module
for that sensor typically sets a DTC with "OAT /
AAT sensor voltage high" in the code description.
The analog voltage you would likely measure if you
back probed between the sensor ground and signal
wire on a voltmeter would be within 1-2 tenths of
a volt from 0 volts or 5 volts if that problem were
currently present. This extreme high or low voltage is called the "dead zone" in any sensor circuit.
These are hard failures. Often times when a sensor value is not an extremely unlikely value such
as an open or short circuit it can still be detected
by a module as inaccurate or 'skewed'. In this case,
you'll read something between 0.02 volts and 4.8
volts back probing the sensor with a voltmeter.
When you use a scan tool to view the sensor value
in degrees of temperature, every bet is off. More recent model year vehicles (since the 2000's at least)
have moved towards more and more sensor default
strategies also known as sensor 'failsoft'. This can
at times, occur whether there is a warning lamp on
the IPC, a DTC in a module or any other obvious
indication of a problem. Failsoft will occur on some
vehicles with hard failures but is very common on
the skewed sensor scenario. Modules view other
sensor inputs with similar functions to perform

what's known as a rationality test. For OAT / AAT
the likely sensors a module would use to perform
a rationality test would be ECT, IAT1 and IAT 2 (if
used) on a cold start. When hard or soft fails occur the module often substitutes a made up value
for that sensor which is at times close enough for
things to work. When the sensor value is in failsoft
mode, a scan tool may display the default substitution value that is close enough to not catch your eye
when you are perusing the PID list on a scan tool.
Frustrating? Indeed - but this MSR will give you a
work around to help you reduce your frustration
and get the vehicle diagnosed properly!
1.
Pull All Module DTCs
First off - scan the entire vehicle for DTCs from all
modules. You may have other sensor related DTCs
that will give you clues to help you troubleshoot
the vehicle. For reading all DTCs in all modules,
you'll need either an "OEM Level" scan tool such
as those made by Snap-On, OTC / Bosch, Launch,
Blue Streak, Matco, Auto Enginuity, etc. or an actual factory / dealer scan tool.
2.
View OAT / AAT PID
With your scan tool connected, go to the IPC /
BCM or HVAC controller with the engine cold and
key on / engine off. Obviously if the DTC was intermittent and there is no problem currently, the
OAT / AAT should read very close (+/- 5 deg. F)
to the actual measured temperature of the shop or
parking lot. If there is a DTC for the sensor voltage being low, this indicates a short or open in the
OAT / AAT, so go to data PIDs and look at the sensor value. As with most thermistors on any vehicle,
- 40 deg. F indicates either an open circuit or the
vehicle's location is in Alaska during the winter
months! If there is a DTC for sensor voltage high,
your scan tool data PID should read somewhere
around 300 deg. F.
OAT / AAT used by PCM?
Go Global OBD II
If there is a DTC and the data PIDs are not near
the obvious extremes of -40 or 300 deg. F look at
the OAT / AAT PID displayed while viewing IPC /
BCM data and then go into the OBD generic / global powertrain side of your scan tool. Even though
the OAT / AAT may be connected to the IPC / BCM
or HVAC module, the digital value for that information is shared on many newer vehicles with the
ECM / PCM via the serial bus. If that data PID for
OAT / AAT is listed in OBD II global / generic the
value displayed should be the actual value the sensor is measuring. For example, if at start up after
several hours of the engine being off, the PCM sees

Dave Hobbs

Figure 5: A 100 ms wakeup signal on a dedicated circuit is sent by the radio to all the modules up to 3X with 2 second delays between each wakeup
signal. If one of the modules doesn't respond, a DTC is set and the radio
sends out a 300 ms wakeup signal. This automatic diagnostic process
takes about 10 seconds. Modules are assigned numbers. The module
furthest upstream from the radio that can still communicate is then
termed a 'surrogate'. A process of using your scan tool on vehicles with
a M.O.S.T. DTC to view which module the radio has assigned surrogate
status to along with the number of that module will give you the location of problems such as the commonly encountered U2098-02 (M.O.S.T.
communications circuit shorted to ground).

March 2017

MACS Service Reports

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017