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Discussion Starter · #61 ·
How the sensor works is also interesting. From factory service manual

Engine coolant temperature (ECT) sensor 2 is a thermistor attached to the radiator. The powertrain control
module (PCM) applies voltage (about 5 V) to the ECT2 signal circuit through a pull up resistor. As the engine
coolant temperature cools, ECT sensor 2 resistance increases, and the PCM detects a high signal voltage. As the
engine coolant warms, the sensor resistance decreases, and the PCM detects a low ECT2 signal voltage.
If the ECT sensor 2 output voltage is more than a set value when the engine coolant temperature is low, the
PCM detects a malfunction and a DTC is stored.

According to the manual, once ECM detects the ETC sensor 2 output volatage is 4.92V or more for more than 2 seconds, it will through a CEL, aka Malfunction Indicator Lamp (MIL). This is also a one drive cycle CEL. And the ignition switch must be turned on. When a malfunction is detected, the MIL comes on and the DTC and the freeze frame data are stored in the PCM memory.

Conditions for clearing the MIL

The MIL will be cleared if the malfunction does not recur during three consecutive trips in which the diagnostic
runs. The MIL, the DTC, and the freeze frame data can be cleared by using the scan tool Clear command or by
disconnecting the battery.

In another part of the factory manual, it is mentioned that this code can be triggered specifically when 'Both fans do not run at high speed with the A/C on (but both fans run at low speed and the A/C compressor operates with the A/C on)'.

I recall that the day when I saw this code it was pretty warm and we turned on AC. Everything was fine. However, just when I stopped the car, the code showed up. I guess without the help of the moving air, the engine decided that the fans did not run fast enough. Anyway, I shall ask my wife to drive with AC on for several days and scan the car again.
 

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Discussion Starter · #62 ·
Wife drove our MDX for several days no issue. The code won't go away. She did not use A/C during this period of time. I am by now pretty convinced that this is not really a ECT (engine coolant temperature sensor). It is more like an A/C coolant temperature sensor. It seems it will only start working when A/C is turned on. If the connection is corroded, or not securely plugged, we will get P2185.

https://www.tsxclub.com/forums/1st-gen-problems-fixes/53563-ect-sensor-location.html
 

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Discussion Starter · #63 ·
I have been using AC recently and the code just wouldn't go away by itself. So today I deleted the code with an ODBII scanner. However, the code almost came back instantly. Then I knew that it was not necessarily all due to AC. So I turned the car on w/o starting engine. Verified that I had 5V voltage at both terminals. Removed and re-inserted the plug and I heard clearly the clicks when the plug went in the sensor. Then I re-scanned the code. Now a warning similar to the one I posted before showed up. The previous one was a red one. The new one is a yellow one for pending code. I deleted the yellow pending code and re-scanned. Nothing showed up. I guess I was too exhausted that day that I did not fully inserted the plug into ECT sensor 2.

Getting into the car, the dash also showed me the same P2185 code and I was finally able to get into the system to view and delete the error message. I guess the sensor and plug made partial contact before and somehow that prevented me from viewing the message. All good now.
 
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