My '99 ML320 has a reoccurring check engine light. The OBDII indicates 2 codes. These are FUEL TRIM BANK 1, and FUEL TRIM BANK 2. Replacing both O2 sensors downstream of the catalytic converters and erasing the codes has not helped. The check engine light still comes on after a short time and the same codes are read by the OBDII. Any ideas on what to do next?
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Fuel trim codes, i.e. P0170 and P0173, have NOTHING to do with the O2 sensors. Depending on which direction the trim has reached its limit, depends on the problem. If it too lean and has reached its rich limit, in other words it can no longer richen the mixture, then you have a vacuum leak [or a bad air mass sensor]. If it is too rich and has reached its lean limit, in other words it can no longer lean the mixture, then you have a leaking pressure regulator or leaking injectors [or a bad air mass sensor]. Understanding what the faults mean and how they happen saves a lot of money instead of using the Mossberg repair technique [loading up the shotgun with a ton of parts and hoping one will fix it].
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