Help with a Long Term Fuel Trim Lean error

[v40alex]

A breakout box is basically a bridge that you put between the plug and connector so you can probe the signal/voltage, etc. while it's all plugged in.

[KBB]

The key to diagnosing rich and lean DTCs if you scan tool supports it is to look at freeze frame data. This is a snapshot of all the ECU key sensor inputs at the time the DTC was set.

The key one to look at is what was the engine RPM (load) when the DTC is triggered. If its at idle speed then a vacuum leak would be the first thing I would be looking at further confirmed by raising the rpm to 2500 and looking at the STFT, if it starts to back off then a vacuum leak is the problem which a smoke test will track down in minutes.

If the freeze frame date shows a high RPM when the DTC is set then this is a lean condition under load and diagnostic's would now focus on MAF and the fuel delivery system.

You can easily test the response of the O2 sensor, we use forced lean and rich tests to confirm O2 sensor function in conjunction with a scan tool. In scan tool live data call up STFT and O2 sensor even better in graphing mode. To force the O2 sensor rich with engine idling introduce propane into the induction system (plumbers torch) or carb cleaner, if the O2 sensor is good you will see a voltage of 0.8v and STFT will go negative.

Next is the forced lean, with engine idling remove the brake booster hose, clamp the open end off and then gradually release introducing the air leak, the O2 sensor will drop to 0.2v and the STFT will go positive.

VADIS and VIDA and indeed many other OEM diagnostic flow charts have very poor diagnostic approaches to fault finding which all too often arrive at the point where they will tell the technician to change the component with no fix in sight. The procedures I mention above will not be found in VADIS or VIDA but will let you see the system respond