Note also that there’s no default strategy for a failed thermostat, which means that the engine and powertrain will react to the actual coolant temperature, rather than a default value substituted by the PCM. The thermostat is considered good if the ECT reaches a minimum of 167º F. According to service information, P0128 indicates that the thermostat is considered defective when the coolant temperature remains lower than its estimated or target temperature. To illustrate, let’s look at the enable criteria for a P0128 cooling system DTC on a 2008 Subaru Outback with the 2.5L 16-valve “H4” engine. Mechanical cooling system failures will usually store a P0128 DTC. MECHANICAL FAILURES Photo 2: This example of a stuck-open thermostat is unusual, thanks to what appears to be a hydraulic spool valve jammed into the valve assembly. In the near future, I’d look for many of these cooling system innovations to be included in stop-start and hybrid engine technology, and I’d also begin to look for many new cooling system malfunction trouble codes. Other configurations use an electric water pump to cool the engine’s turbochargers after a hot shutdown. Some cooling systems use electric water pumps to circulate warm coolant through one or more heater cores to keep passengers warm when the engine is shut off. Some cooling systems include features like electronic thermostats or even dual thermostat configurations, all of which are designed to shorten engine warm-up times. On other platforms, water-cooled exhaust manifolds force engines to reach operating temperature within minutes of a cold-soak start. As a result, we might be seeing old-technology components like radiator shutters make a comeback on some vehicle platforms. Modern cooling system design is most often concerned with warming up the engine as quickly as possible and maintaining a constant coolant temperature. So, unless the ECT achieves full operating temperature within a specified length of time, diagnosing an intermittent or random no-code driveability or stalling complaint is nearly impossible. For example, a DTC for a failed oxygen sensor might not be stored if the engine fails to reach operating temperature. While enable criteria might include engine run time, specific system voltage levels, engine load and other operating parameters, most enable criteria will also include engine coolant temperatures. When designing a diagnostic test monitor, automotive engineers create a set of operating conditions (enable criteria) that must exist before the monitor will test the circuit in question. Photo 1: Will this 2002 Toyota 4Runner store DTCs at 178º F coolant temperature? The answer lies with the 4Runner’s DTC enable criteria.
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