Sometimes when a trouble code is stored in the engine module on an OBDII vehicle there will also be a "snapshot" of the sensor readings at the time the code set in the system. Codes get different priorities and some more recent freeze frames may replace older ones. There is also that the system is basic (global) OBDII and the sensor data is pretty limited. With that disclaimer mentioned, you can still sometimes get a clue to what may be causing a problem. So it is worth a look if your tool supports it.
We began with a basic OBDII code reader and the next step up will be OBDII code reader with freeze frame. For an example, here is a 2006 Toyota Matrix with a system lean code P0171. A P0171 is a trouble code that will set if a fuel mixture is too lean as indicated by the O2 sensor. Not enough fuel or too much air can set the code, if the system is working properly.
If the condition appeared at idle or low rpm you can expect it to be too much air. A vacuum leak. You don't have a lot of fuel demand under those conditions but you do have a lot of engine vacuum. If the condition appeared at higher demand conditions it is more likely to be a fuel delivery problem. Freeze frame may show what I need to know.
When you look at freeze frame data, look that the data applies to the code you are diagnosing. Also, don't get caught up in data that means nothing. You only need a few relevant items.
We have a light load at lower rpms on a cold engine and a LOT of fuel being added to the mix by the computer via fuel trims.
When you put this combination together, the lean code, the high fuel trims, the cold engine vacuum leak at low demand................... you think there may be a vacuum leak under the intake manifold. It is very common for the seals under the intake to draw up (shrink) a bit when cold and allow air to be drawn in. As the engine warms the seals usually expand and return to normal sealing. To verify, I pulled up short term fuel trim as a data item and graphed it. I took a can of my favorite, Berryman's Spray Carb cleaner and sprayed a little at the base of the intake ports while the engine idled. If the spray was drawn in then I had a vacuum leak at the seals. A quick way to see if this happened is to watch the fuel trim. If the cleaner was pulled into the intake the car would react as though it was more fuel and take fuel away from the mix. A quick negative fuel trim. Like this:
The cause of the P0171 then is bad intake manifold seals. It was a lean fuel system code. It wasn't a bad intake manifold seal code. You have to trace things to the problem with logic and testing. Whats up next? Graphing maybe?
Kenneth Hayes
G&G Auto Repair
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