I been get heavy into the scanning tools lately on my Ford van (98 e150 5.4).

Anyhow... I'm layin into it deep like all these readins of temperatures and such.

For example, I see "head cylinder temp" and it says 422 for one of 'em ... and 190 for the other one (I'm assuming there are two because there are two different readings... in farenheight I suppose)... and that gets me worried. That seems awful high.

But do I really need to worry about it? There is a check engine light after all... and it is nice and quiet.

Or let's say transmission fluid temp. It says 167. Is that too high? Do I have to look up every reading on the internet to understand it and check it every time I drive?

Half o me says hey just don't worry about all that stuff cuz it were outta the acceptable ranges the check engine light would come on for ya.

But I know for a fact the check engine light don't always come on, like lets take a simple overheat situation. Unless you got your eyes glued to the dash, the first sign o' that you gown get is pop, a hiss, and plenty o steam blindin you as you veer off the highway.

I think that's unsatisfactory. I think there outta be a coolant flow sensor (by amount per unit of time)... and I think that there outta be some kinda extreme buzzer goes off like in a airplane when it's flying to low. This is serious stuff.

So that's why I don't trust the check engine light no more.

So I guess what my specific question is, so I can avoid getting this closed again... is...


what is the specific standard on odb-ii that determines what triggers a check engine light and what don't?

Thanks y'all,



2 Answers 2


So I'll start by answering your question listed at the end. First off, while there is a small level of standardization amongst OBD2 reported codes, there isn't standardization as to what sets these codes. This information, generally know as the code setting criteria, is written by the manufacturer of the unit - say the engine. This criteria will differ as system designs vary wildly between manufacturers and even models. Anyways, The computer looks for specific values and runs specific tests at specific times to determine whether or not the system is either operating as it should or if a light needs to be set. In short, a code is set if the computer determines that any system isn't operating within desired values. For example, one way a computer MIGHT determine if a misfire is present is by monitoring the period of time between each tooth of a wheel mounted to the crankshaft and looking for deviation or extended periods of time between pulse.

Another note, there is also criteria for turning off the check engine light - removing codes. These are separate from the criteria to set it but are generally related.

As far as, reading the individual temperatures and values, don't bother unless you are attempting to diagnose a problem. If anything, reference factory service data on what the reading should or shouldn't be. Keep in mind, however, that temperatures/values can vary wildly depending on the conditions the vehicle is exposed to and even the weather/atmospheric conditions.

As a wrap up let me start by saying this in the most honest way I can - DO NOT start poking and prodding into territory you don't fully understand. The people that originally designed and engineered the vehicle knew what they were doing. Now, no vehicle or human engineer has or will ever be perfect but they did know what they were doing. Instead of reading values and questioning each and every one until you drive yourself insane and mad try this instead:

  • If you aren't already familiar with the basic operation of your vehicle and it's systems then educate yourself on it. It will pay off more and more if you understand even the basics. There are so many way to do this now-a-days and lots are free.
  • Perform maintenance on your vehicle. Keep logs and read them. If you're worried about coolant/overheating issues then inspect your hoses and coolant. Check for leaks. The last thing that is going to help diagnose coolant issues is going to be a cylinder head temperature reading. If you aren't comfortable or familiar with these system then have someone professional take a look.

Code readers and scan tools are great and I'd recommend that everyone who has a vehicle has one. A scan tool, however, does not replace basic inspections and maintenance.

P.S. Based on your question I made some assumptions regarding your knowledge and skill level. If I am incorrect in my assumptions then I apologise and I would be happy to go further in depth or explore other questions you may have. Despite how this may read at times, my only intention here is to be only helpful. Regardless, I wish you the best of luck!!

  • 1
    There is a general principle that is applied to medical diagnosis: if you don't know in advance what you are going to do with a test result, don't do the test. The same principle applies to measuring engine parameters with an OBD scanner!
    – alephzero
    Jun 19, 2020 at 2:53

For example, I see "head cylinder temp" and it says 422 for one of 'em ... and 190 for the other one (I'm assuming there are two because there are two different readings... in farenheight I suppose)... and that gets me worried. That seems awful high.

So, just taking this one example of yours, we will need to know exactly where the two sensors are located. One sensor could be located close to the combustion chamber for testing purposes and the engineers use that when checking how the engine is running.

The other sensor (a reading of 190) is likely to be located close to, or even in, a water channel in the cylinder head perhaps to give the water output temperature.

The engineers may well have a set of data they can refer to (based on running engines on a test bed in controlled conditions) where the difference between the two readings can be compared to determine how the engine may be performing at a given time, this will be used in conjunction with other readings as well.

So, if you go trawling through all the possible readings then you do need to make sure that you know where the readings are being taken from and under what conditions.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .