Bad spark plug wires (4.6L equipped with coil packs).This will have a direct and negative impact on the PCM's fine-tuning of the fuel injected into the engine. A misfire condition will cause the air/fuel mixture to go rich. Verify that you do not have a misfire condition. What you're looking for is anything that would add fuel to the air/fuel mixture beyond the control of the PCM.
OK, after confirming that you do have a bonafide rich condition on one or both banks, you can start doing some specific tests to find out what's causing it. This also means that whatever is setting these codes is failing intermittently and intermittent failures can not be diagnosed as long as the problem is not present. Why? Well, it all depends if one or both banks are being affected.ĬASE 1: Your scan tool showed one or both LT FTRMs at -20% This tells you that there truly is a problem causing the PCM to cut as much fuel as possible to that bank or those banks.ĬASE 2: Your scan tool showed one or both LT FTRMs at between 10% and -10% These are the normal values that a vehicle without any fuel trim problems should have. You may or may not see them both at -20% and this is OK. If the rich condition is present, that's causing the ‘Fuel Trim Too Lean’ code, the LT FTRM1 and/or LT FTRM2 will be at -20% (that's negative 20%).
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