MACS Service Reports - 2016 - JUL8

What Can Set a P0128?

To help, the ECM has two strategies which are
used to prevent a false thermostat diagnosis. For
vehicles with dual heaters, the WUT threshold is
reduced at cold ambient temperatures below 50
deg 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 thermostat diagnostic does not make a
fault determination for that driving cycle, i.e. "nocall".

The ECM determines that the modeled ECT has
met the calibrated target temperature of 92°C
(198°F) and the measured ECT is less than 79°C
(174°F) for greater than 2 s.

Action Taken When the DTC Sets
* DTC P0128 is a Type B DTC.
* The Engine Coolant Temperature
Gauge is inoperative.
* The cooling fans will be commanded
ON.
* The AC compressor will be commanded OFF.
DTC P0128 is a Type B DTC.
The WUT (Warm Up Temperature) threshold is
normally set to 20 degrees F below the thermostat
regulating temperature. Two common issues can
lead to a false condemnation of the thermostat. One
condition is when the heater is pulling out more
heat than the engine is creating. For example on
large passenger vans with front and rear heaters in
very cold conditions, even a good thermostat may
never warm up to operating temperature. Another
example is a vehicle that is started and idles for a
very long time with the heater on high and the defroster fan on high.

Things to Check First!
1. DTC P0128 poor heater operation
could be low coolant level or a bad thermostat. Check the coolant level first.
2. ECT inaccuracy due to corrosion on
the sensor terminals, corrosion on the
sensor 's sensing element (really bad
coolant) can trigger a false thermostat
DTC.
3. A bit of extra resistance in the ECT circuit can lead to false thermostat DTCs.
4. DTC P0128 and DTC 2181 both currently failed could indicate a stuck open
thermostat. ■

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February 15 to 18.

MACS Service Reports is published monthly by the Mobile Air Conditioning Society Worldwide. It is distributed to members of MACS Worldwide
and is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will be
considered for publication, however, MACS Worldwide reserves the right
to choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Forcey
Manager of Service Training:
Steve Schaeber
Mobile Air Conditioning Society Worldwide
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsw.org * Website: www.macsw.org

Unless otherwise noted, all photos/art by author.

July 2016

MACS Service Reports

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