Just installed a Scanguage 3 on my truck. Primarily want to monitor coolant, engine oil, and transmission fluid temperatures. How accurate are the OBD 2 readings?
well, I guess it would be useless if it was not accurate. 
They are as accurate as the sensors in your car. Not lab grade. +/- 5 % is about what you can expect.
Keep in mind, some of those sensors are faked with software.
Perfectly accurate for use as a gauge since you don’t need lab grade accuracy to monitor the car.
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Exactly. The sensors input data to the ECU. The ECU uses that data to make the engine run smoothly and it’s optimal performance. If it’s not then the vehicle won’t run properly or even run.
I’d be surprised it’s that high. 5% difference can make a huge difference in engine performance.
Temps sensors are often thermistors, pressure sensors are less than lab grade. Both are a bit non-linear but that can be adjusted in software for better accuracy… half or less of the 5%.
Consider old style O2 sensors that were wildly non-linear but still capable of being filtered to get a decent signal for the ECU… but they aren’t very good. Many cars now have wideband O2 sensors which are far more accurate.
But much more expensive… that’s the reason low resolution sensors are used. Also the reason software driven estimations are used.
My Ford has a cylinder head temp sensor but no coolant sensor. Head temp is apparently more valuable to Ford to read temps. The government mandated universal set of OBD outputs requires a coolant temp reading…so software provides that. Most drivers understand coolant temp more than head temp so it feeds that gauge, too.
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Chances are your truck doesn’t have sensors for oil and transmission temps
And as usual nothing more descriptive or specific than “my truck”
Person has a Nissan Frontier truck ( year unknown - engine unknown ) and buys a product then goes on the web asking strangers if the product is reliable . OK Fine 
The truck is a 2020 Nissan Frontier crew cab 4X4 SV with a 3.8 V6 and 9 speed automatic transmission. The question was not about the reliability of the product but about the accuracy of the data coming out of the OBD 2 port.
Thanks to those with helpful replies.
As new as it is, it just might.
I do get readings for oil, coolant and transmission temperatures.
My 1996 Dodge is new enough to have a transmission fluid temperature sensor.
All three should be within 2 to 3 degrees which is more accurate than you need. Coolant temps vary over a 10 degree span given thermostats and cooling fans. Oil temps can spike at times by 20 degrees and more without any issues.
Just note temps over a range of traffic. If you are trailering, note the differences between driving in traffic and trailering on the highway. That will guide any choice of additional coolers.
Is your plan to plug something into OBD II port full time, as your drive, to obtain & display this sensor info? If so, be aware this may overload the drivetrain computer system, and cause unanticipated problems. OBD II systems are generally not designed for a real-time monitoring applications.
Remember the first moon landing in 1969? They had a similar problem, computer overload caused by radar sensor monitoring, almost caused an aborted landing.
Just reading the data on the stream won’t affect anything. I use these devices a lot recording far more than 3 outputs.
It does. xx
I’ve had a ScanGauge II installed in my '19 Frontier for over a year. Very pleased with it, but I haven’t done the programming for transmission temperature yet.
Didn’t know there was a ScanGauge 3.
I don’t know if the 19 Frontier has an ATF temp sensor but the 20 does. The 20 got a JATCO version of the 9 speed designed by Mercedes. To fill the ATF, you have to monitor the ATF temp.
It apparently does, but I haven’t verified it by programming my SG II to read it.
It makes sense. With diminishing returns on efficiency and emissions improvements, every little bit counts. The coolant has way more hysteresis than the head when responding to thermal changes. They want to start dialing back the fuel injection ratio as soon as possible in response to rising combustion chamber temps when the engine is cold. The head provides a faster response and is more accurate at reflecting the true combustion chamber temp than a coolant sensor located some distance away with cycling coolant… if one is to be a best guess, better the coolant than the head
My TB has an oil pressure switch and an analog gauge. If the pressure is sufficient to activate the switch, then they calculate what the pressure would be based on engine RPM to feed the analog gauge and make it appear as though it is directly being read. Not a big fan of this…
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