MACS Service Reports - 2016 - JUL5

rounding the ECT due to an air lock or low coolant
level. A DTC may or may not set in either case. A
failure that has been seen in the field with some degree of regularity is an open in the sensor element
internally. This can be intermittent. Even though
the circuit for the ECT draws very little current, a
voltage drop test can be performed in the case of an
intermittent P0118 or P0119. (Fig. 7)

when there is a short in the ECT sensor signal wire
to ground. In this case DTCs P0117 & P0119 should
set. When there is an open / high resistance in the
signal wire DTCs P0118 or a P0119 when the sensor puts out a voltage that is intermittent). Those
same DTCs will also set when a short to voltage is
detected by the ECM. If the signal wire is shorted
to true B+, the PCM may be damaged. Remember,

Figure 7: A load substitute should be used when performing a voltage drop of suspect sensor circuits. The ECM and the ECT MUST BE DISCONNECTED
for this test. Measure the voltage between points 1 and 2. If that voltage is no lower than 0.5 volts of the voltage measured directly at the power source, the
circuit is fine. If that voltage is more than 0.5 volts lower than the source voltage, you have an excessive voltage drop in the harness or connections somewhere. To narrow it down between the sensor signal circuit and ECM provided ground, measure between points 1 and 4 and then points 2 and 3.

DTC P0128 - Thermostat Faulty? Maybe

analog (continuous and variable) sensors typically
have 5 volts running to them. (Fig. 8) ECMs for
many vehicles built in the last few years may have
multiple 5-volt reference power supplies internally
to prevent the loss of all sensor circuits in the event
of a short to B+ or ground. This prevents everything in the way of engine management inputs being lost due to a single problem on a sensor circuit.
The ECM's dedicated ground to all sensors (Ref
Low) can go open circuit or have high resistance in
it and set the same P0118 and P0119 as a problem
with the signal wire.

The "0" after the "P" in this DTC indicates that
this code can set in a lot of different makes and
models of vehicles as opposed to a DTC that may
only apply to a particular OEM as would be the
case if the "P" for Powertrain preface was followed
by a "1" instead of a "0." This DTC is officially described with more complex wording like "Engine
Coolant Temperature (ECT) Below Thermostat Regulating Temperature" in the case of GM or simply
put "Thermostat Rationality" in the case of Chrysler.

ECT Sensor Troubles?

ECT Sensor Grateful "Dead Band"

This DTC can flag when there is another problem besides the actual thermostat including other
DTC issues. It's important to really know your ECT
sensor DTCs when attacking a stat DTC. As for the
P0128, it can be accompanied by other DTCs such as

If the ECM, or any other electronic module for
that matter, sees a signal of 0.2V or less from a sensor, the control module will identify this as an outof-range signal and will suspect that the circuit is
either shorted to ground or a loss of voltage has

July 2016

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016