Considering that roughly 1,500 or more different vehicle
models are introduced into our domestic market each year, it’s becoming more
difficult to predict how a Powertrain Control Module (PCM) will utilize data
from a particular sensor or detect an out-of-range sensor in any single vehicle
The engine coolant temperature (ECT) sensor provides a good
illustration of how many on-board diagnostic strategies have changed. Keeping
in mind that an of out-of-range ECT sensor can, among other things, affect the
PCM’s fuel and spark mapping, variable camshaft timing, transmission, radiator
cooling fan and evaporative emissions functions, it’s important to develop an
awareness of how the PCM self-diagnoses the ECT circuit and how the ECT data is
integrated into a vehicle’s operating strategy.
Much of any on-board diagnostic strategy depends upon the
computing capacity of the PCM. Most pre-1996 OBD I and many early post-OBD II
Engine Control Modules (ECMs) and PCMs had only enough computing capacity to
detect hard or intermittent circuit voltage faults.
In many cases, early ECMs didn’t have enough capacity to
rationalize the performance of the ECT sensor with other data inputs. So, in
some applications, it’s possible that an out-of-range sensor can affect the
operation of many OBD II test monitors and the operation of many vehicle
components without setting a trouble code. In passing, remember that the ECT
input is part of the freeze-frame data that accompanies most diagnostic
Diagram 1: The coolant temperature should rise steadily as the engine warms up.
Most modern automotive ECT and intake air temperature (IAT)
sensors are generally two-wire, “negative temperature coefficient” (NTC)
thermistors in which the electrical resistance of the ECT and IAT sensors
decreases as temperatures increase. See Diagrams 1 and 2.
Diagram 2: The coolant temperature should level out as the thermostat opens.
At the extremes, an open-circuit ECT should indicate a scan
tool data of approximately -40°F coolant temperature, since the PCM is
receiving a zero return voltage. In contrast, short-circuiting the ECT
connector from the PCM’s 5-volt reference terminal to the PCM’s voltage return
terminal should indicate a scan tool data of approximately +300°F coolant
Both temperatures are programmed into the on-board
diagnostic strategy as the most extreme temperatures under which the engine
might be expected to operate. The first series of “Global” circuit-related
codes include P0115 (ECT circuit fault), P0117 (ECT low input voltage), P0118
(ECT high input voltage) and P0119 (ECT sensor or circuit erratic).
Electrical failures include low ECT return voltages caused
by corroded ECT connectors or, at another extreme, a low reference voltage
caused by another sensor shorting the reference voltage circuit. In some cases,
a P0116 DTC will be set if the PCM detects an error in the range or performance
of the ECT sensor.
Mechanical failures include low ECT return voltages caused
by low coolant levels and stuck-open thermostats, which are often represented
by a second series of P0125-128 DTCs. The low coolant level will cause a much
lower-than-expected ECT return voltage because the ECT sensor is no longer in
contact with the coolant.
Presumably, the driver will see a “low coolant” warning
light on his instrument cluster. Perhaps the “Check Engine” light will be
illuminated and a DTC set, or perhaps not. In contrast, the stuck-open
thermostat will cause a slow warm-up time and might store a P0128 DTC simply
because the PCM sees a lower-than-normal coolant temperature for a
predetermined length of time.
Because the ECT sensor is a primary input data, practically
all ECMs and PCMs are programmed to detect open and shorted circuits in the
ECT circuit. But, when detecting an out-of-range ECT sensor, the actual ECT
test monitor can vary among applications. The PCM can, for example, measure the
time, speed and load required to bring an engine up to a predetermined coolant
temperature of, let’s say, 194°F.
If the indicated ECT data hasn’t reached the desired
operating temperature during a specific time limit and at a specific engine
speed and load factor, the PCM might set a P0125 (insufficient temperature for
closed-loop operation) or a P0128 (coolant temperature below
thermostat-regulated temperature), which in most (but not all) cases indicates
a bad thermostat. If this diagnostic strategy sounds complicated, that’s
because it is complicated, and also because it can vary widely among different
Enabling criteria are simply the types of sensor inputs
required by the PCM to run a test monitor and to set a specific DTC. Since
enabling criteria are application-specific, an appropriate technical database
must be consulted before making any assumptions. The engine coolant temperature
is important because it forms part of the enabling criteria for many component
test monitors and is part of the freeze-frame data for most DTCs.
Diagram 3: This sharp drop in the ECT signal caused the air/fuel mixture to momentarily lean out, which caused an intermittent, no-code stalling complaint on this 1997 Toyota Camry. This very same driveability complaint might not exist on a 2013 vehicle.
As illustrated above, if the ECT sensor is indicating a
momentary dip in coolant temperature on a 1996 OBD II vehicle, the result might
be a no-code, cold-engine driveability complaint because the PCM has increased
fuel delivery to meet the fuel map for the indicated (not the actual) engine
operating temperature. See Diagram 3.
If the ECT is indicating a lower than actual operating
temperature, it’s possible that the PCM might increase the pulse width to
enrich the fuel mixture only until the oxygen sensor provides a data input to
the PCM so it can assume fuel control. With early OBD II vehicles, an over-rich
condition might also depend upon how much authority software engineers
programmed into the PCM for the ECT input. On low-authority systems, the
effects would be negligible, whereas on high-authority systems, the effects
might be profound.
OLD VERSUS NEW
But let’s fast-forward to 2013 when a vehicle has a far
greater capacity to detect a sensor fault than does the PCM in a 1996 model.
Here’s where experience can lead us astray. For example, a 1996 engine might
compare the data inputs from the IAT sensor and the ECT sensors to determine if
the engine is starting from a cold-soak or a hot-soak condition. If both
temperatures are within, let’s say, eight degrees of each other, the PCM
assumes that the engine is starting from a cold-soak condition. This data
allows the PCM to adjust the spark and fuel maps to start and run from a
But, let’s say that the ECT resistance is lower than
specification and is therefore indicating a higher coolant temperature. In this
case, the PCM might assume that the engine is starting from a hot-soak
condition, when, in fact, it is not. This false data might cause a cold
driveability complaint, and, among other things, possibly prevent the
evaporative emissions monitor from running.
With 1996 vehicles, it’s also conceivable that an out-of
range ECT sensor or stuck-open thermostat can prevent a DTC from being set for
a defective oxygen sensor because the system never reaches closed-loop
operation. Similarly, many 1996 automatic transmissions might not engage the
torque converter lock-up clutch or transmission overdrive gear until the ECT
sensor indicates that the engine has reached a specific operating temperature.
On the other hand, because modern heated zirconia oxygen or
air/fuel ratio (AFR) sensors on a 2010 vehicle allow the PCM to assume fuel
control practically as soon as the engine is started, the oxygen or AFR sensor
is given more authority than the ECT sensor for entering closed-loop operation.
Multiple A/F and oxygen sensors also provide a backup data stream and allow the
PCM to compare the data inputs of each sensor.
So, an out-of-range ECT sensor on a 2010 vehicle would
likely not affect driveability or performance as much as on a 1996 model.
Instead, the 2010 PCM might project a value for the expected engine
temperature by monitoring enabling criteria like intake air temperature
engine speed and engine load. Furthermore, the additional computing capacity
of the 2010 vehicle might allow its PCM to overlook a momentary glitch in the
ECT data input (See Diagram 3 on page 28) and instead simply store an
ECT-related trouble code in its diagnostic memory.
BASIC ECT DIAGNOSTICS
The simplest diagnostic strategy for diagnosing IAT and ECT
sensors is to compare their data inputs after the vehicle has cold-soaked
overnight. A second strategy can include using a scan tool to graph the ECT
voltage. A third, but less reliable, method is to use an infrared pyrometer or
“heat gun” to compare both intake air and engine cylinder head temperatures
with the data stream displayed on a scan tool. But, remember that due to the
“reflectivity” of various surfaces, the heat gun approach will not indicate
the exact temperature indicated on the scan tool.
Lastly, make sure you’re testing the correct sensor. Keep
in mind that the IAT sensor is usually integrated with the hot-wire mass air
flow sensor assembly on most current vehicles. Many pre-1996 OBD I vehicles
included a separate temperature sensor for activating the radiator cooling
fans. Early OBD I and OBD II vehicles used a single-wire ECT sensor to supply
data to the instrument cluster temperature gauge and a separate two-wire
sensor to supply data to the PCM. Thanks to multiplexing, which makes it
possible to share a single datastream among various control modules, modern
vehicles generally use a single ECT sensor to supply engine coolant
temperature data to various modules.