MACS Monthly Newsletter - 2020 - APR5

Ford's Summary for Gasoline Engines

For the 2009 MY, the thermostat test has been
enhanced to reduce the time it takes to identify a
malfunctioning thermostat. The enhanced monitor includes a model which infers engine coolant
temperature.

Ford

During a cold start, when the thermostat (Figure
10) should be closed, the monitor uses a model of

Figure 12: Ford's document lists 300 seconds (5 minutes) as
the time involved for the P0128 monitor.

Ford

The WUT threshold is normally set to 20°F below
the thermostat regulating temperature.
There are some circumstances that could lead to a
false diagnosis of the thermostat. These are conditions where the vehicle cabin heater is extracting
more heat than the engine is making. One example
where this can occur is on large passenger vans
which have "dual" heaters, one heater core for the
driver and front passengers and another heater core
for the passengers in the rear of the vehicle. At very
cold ambient temperatures, even a properly functioning thermostat may never warm up to regulating temperature. Another example is a vehicle that
is started and simply sits at idle with the heater on
high and the defroster fan on high.

Figure 10: Ford's OBD-II Summary Document. Each of
Ford's model years from 1996 to the current model year has
a document (free to download from motorcraftservice.com)
that goes to great lengths in describing the entire engine
and transmission control system to nearly an engineering
level. Here is a simple block diagram leading up to more
detailed information on DTC P0128.

Ford

ECT to determine whether actual ECT should have
crossed the WUT (warm up temperature) threshold
(Figure 11).

There are two features that are used to prevent a
false thermostat diagnosis. For vehicles with dual
heaters, the WUT threshold is reduced at cold ambient temperatures below 50°F. For cases where
the engine is not producing sufficient heat, a timer is used to track time at idle or low load conditions (e.g. decels). If the ratio of time at idle/low
load versus total engine run time exceeds 50% at
the time the fault determination is made, the therFigure 11: Temperature increase over time. Chart showing the mostat diagnostic does not make a fault determiWUT (warm up temperature) threshold.
nation for that driving cycle, i.e. "no-call".

Once the ECT model exceeds the WUT threshold,
after a calibratable time delay, measured ECT is
compared to the same WUT threshold to determine
if ECT has warmed up enough. See Figure 12. If the
ECT has warmed up to at least the WUT threshold,
the thermostat is functioning properly. If ECT is too
low, the thermostat is most likely stuck open and a
P0128 is set.

April 2020

Reading through the details in the Ford OBD-II
document, it should become apparent how important an accurate ECT sensor is to the ECM for this
monitor to pass. In the 2012 Equinox case study
Mike did his homework and replaced the sensor
along with the pigtail connector to the sensor. It's
important to remember that sensors can be skewed
(moderately inaccurate) and not necessarily set a

MACS Service Reports

http://www.motorcraftservice.com

MACS Monthly Newsletter - 2020

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