Auto Service Professional

FEB 2016

Magazine for the auto service professional

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53 | February 2016 another local shop and the customer tells a story of many dollars being spent, the CEL still being on, but also adds that the vehicle runs and drives just fne. He just wants the CEL light off so that it will pass its emission test. The frst thing I did was look over the invoices from the previous repairs to gain some insight into what's been done; in this case the vehicle kept setting a P0171 lean code. The other shop had replaced the MAF sensor, changed the fuel pump, fuel flter, plenum gasket, air intake snorkel and the A/F sensor. But after all these repairs the P0171 would return within a day with the CEL on. After a thorough visual inspec- tion that didn't expose anything obvious, I plugged in my scanner to see what codes were set, and it was a P0171. I looked quickly at the freeze frame data and it defnitely showed that the vehicle was adding fuel (I also noted that the code set at a low rpm). I wrote down the MAF sensor's part number to have our dealer verify that it was the correct model for this engine combina- tion (Bosch made a number of MAF's that look the same and ft just fne, but have different calibrations). After a quick call, I confrmed it was the correct MAF sensor, so I needed to look elsewhere. I traditionally check for CEL codes using OBD-II: it's the system that turns on the CEL light, so I trust what it tells me. But I wasn't getting anywhere quickly with the data OBD-II was giving me, so I installed my VAG COM scanner to see if there were any specifc VW codes that would help me in any way. There were two VW-specifc codes, 16555 and 17559. The 16555 is the VW equiva- lent of an OBD-II P0171 and the 17559 translates to an OBD-II P1151. Both these codes pointed to the fact that this engine was experiencing a lean condition, but the second VW code 17559 also told me that the lean condition was only at idle. Most European cars express fuel trim values in two terms: additive or Idle O2 Adapt (the additive value is the fuel trim variation at idle either adding or removing fuel) and multiplicative or Run O2 Adapt The Tech Bench There were many new parts, such as this MAF sensor and it had the correct part number. These VWs have robust EVAP fttings and connectors.

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