I got some data on sending 0202 command (Mode $02, PID 02). It is the freeze frame data. I don't understand why does anyone need the data? Do anyone (including mechanics) ever open Freeze frame data? What will one do knowing that when MIL appeared, the car speed was 0 kmph, rpm was 1200 rpm, AFR was 14.7 etc. Can I conclude anything from the freeze frame data that I get that would help me diagnosing the fault code that appears? I am really unable to understand the importance of the data. Can someone please help me understand if the data is at all useful.
While the fault code that popped the MIL is tremendously useful by itself, freeze frame data offers huge help, as it allows you to see the exact conditions under which the engine decided that there is a fault. You will know at what RPM the code was popped, what the engine load was, whether the engine was warm and in closed loop mode, if it was overheating, whether the air pressure/volume was normal, what the fuel trims were, etc. All of this can save a lot of headache to the mechanic who is diagnosing a fault that isn't obvious or is intermittent.
Let's say you have a cylinder misfire code. Well, the engine misfires, not very helpful, is it? However freeze frame data will help you determine what exactly was going on when this happened - for example if it only happens under maximum throttle and load then the reason for it could be completely different than would be at idle.
They help us home in on what might be the root cause
Maybe an example would help explain why freeze frames are so useful
Here is the
P0300 freeze frame from that infamous Merc GLK280:
+-----------------------------------------+-------------+ | Fuel System 1 status | 1 | | Fuel System 2 status | 1 | | Fuel System 1 status | 1 | | Fuel System 2 status | 1 | | Calculated load value | 23.14% | | Engine coolant temperature | 49 C | | Short term fuel % trim - Bank 1 | 0% | | Long term fuel % trim - Bank 1 | 11.72% | | Short term fuel % trim - Bank 2 | 0% | | Long term fuel % trim - Bank 2 | 7.03% | | Intake manifold absolute pressure | 28 kPa | | Engine RPM | 1293.75 RPM | | Vehicle speed | 17 km/h | | Ignition timing advance for #1 cylinder | 42.5 deg | | Intake air temperature | 46 C | | Mass air flow rate | 5.63 g/s | | Absolute throttle position | 13.33% | | Time since engine start | 26 sec | | Fuel rail pressure | 380 kPa | | Commanded evaporative purge | 0% | | Fuel level input | 45.49% | | Barometric pressure | 99 kPa | | Control module voltage | 13.29 V | | Absolute load value | 17.25% | | Fuel/Air commanded equivalence ratio | 1.54 | | Relative throttle position | 1.96% | | Ambient air temperature | 36 C | | Absolute throttle position B | 12.94% | | Accelerator pedal postion D | 6.27% | | Accelerator pedal postion E | 6.27% | | Commanded throttle actuator control | 2.75% | +-----------------------------------------+-------------+
And here are a few of the questions that I was trying to answer when digesting the freeze frame data:
"Hmmm, the car's misfiring. That usually means a lean mixture, which could be caused by a whole bunch of things"
- Under what circumstances did the CEL get triggered? The engine was running for just 26 seconds, so is there a cold-start issue going on here?
- Is there a lack of signal from a vital sensor like MAF/MAP?
- Was the car in open-loop mode or closed-loop mode when the CEL appeared?
- Was there a real lean condition, or does the engine management think that there is a lean condition?
- How was fuel pressure? Is it lower than expected? That could indicate a possible fuel starvation issue.
- Why are those fuel trims so high? Is there a vacuum leak that we need to hunt down?
- The calculated load value is low, so the leak should be relatively easy to uncover.
The freeze frame data by itself won't be of much use without someone interpreting the data. As with all tools, its usefulness will be determined by how it is used.
We find FF so useful that we retain it on the customers invoice in case the same code sets later. It is also very helpful when used in conjunction with a test drive on intermittent faults. It tells exactly how to duplicate the conditions under which the fault set. There are times when its limit of setting on only one code is frustrating because it is not always available on the code that is of the most interest.
An example: It makes all the difference on Honda V-tec codes. If the code set at an rpm below normal on command we know the problem is with the fault feedback system not the system itself. If the code sets above normal actuation rpm we know to test the v-tec actuation system. FF is often the way we know what part to replace because these faults are so intermittent.
About 80% of the time it is not needed. But when it is needed it can make all the difference between a happy customer and a comeback.