Last month a customer brought in his 2013 BMW 335i 3.0-liter sedan because the service-engine-soon light that appeared on the dashboard prevented the car from passing an emission test.

The customer told us the car had been driving well and did not exhibit any drivability problems. He also said the warning light had only been on for about a week.

We connected the car to the Intelligent Service Technical Application and scanned all the modules. As a result, we pulled several codes from the BMW Digital Motor Electronics: 002A00 (secondary air system), 0039F4 (catalytic-converter conversion), 002A00 (secondary air system), 002A13 (diagnostic module tank-leakage, or DMTL, activation), 002E68 (knock sensor signal 1) and 0029E1 (mixture control 2). Of these codes, two were active – those pertaining to the secondary air system and the evaporative system.

We then collected the environment information for each code and ran the test plans. The secondary air system passed its functional test but then set off the code again after the test was completed. The evaporative system could not finish the functional test, and the DMTL pump failed in the test plan.

I have written about these types of systems in the past. Evaporative fuel systems use a number of components (valves, plumbing, charcoal canister and a leak detection pump) to check the integrity of the fuel tank and its components. It does this so unwanted fuel vapors (hydrocarbons) do not escape into the atmosphere. The secondary air system also uses a number of components (air pump, plumbing, valve, exhaust components and sensors) to divert fresh air into the exhaust to warm up the catalytic converter when the car is cold.

Tackling the problems

After running the test plans, we talked to the customer and decided to address the problems one by one. We first confirmed a leak in the DMTL pump and replaced it. After that, we ran the test plan on the evaporative system and everything passed. Things were going smoothly until we got to the secondary air system code.

This is where it gets interesting. We ran the test plan on secondary air and saw that all of the components were doing what they are supposed to do. At the end of the test, it indicated that the system failed, but it did not give any direction, such as “secondary air pump short circuit to ground.” This code simply indicated that there was a problem somewhere in the entire secondary air system. While it’s rare for a code to tell you what the exact problem is, this at least gave us some idea.

We then took a step back and started testing each part of the secondary air system. We commanded the secondary air system to turn on with the scanner (ISTA), so we knew the control worked. We tested the secondary air pump and the combination valve, checked for a carbon block in the cylinder head to the exhaust and tested the front and rear oxygen sensors. Everything tested fine.

Finding the fix

So, if everything is OK, then what’s causing the problem?

The secondary air system pumps fresh air into the exhaust through the cylinder head to warm up the catalytic converter faster. One thing that could cause that air flow to have less volume is carbon blockage in the cylinder head, but we had already confirmed that was not the case.

We then went back and looked at the whole picture. After reviewing the codes again, we locked onto 0039F4 – catalytic-converter conversion. We had not been focusing on the catalytic converters because they had not failed during all the tests, and the car was not exhibiting any type of catalytic symptom (such as rattle, misfire due to catalytic blockage or low power). That got us thinking that if the catalytic converters were just slightly plugged, they could be restricting the secondary air system flow. We installed a back-pressure gauge in the exhaust system and found that the catalytic converters were about 15%-20% plugged.

We then made an exhaust leak to test the theory. Once we relieved the back pressure, the secondary air system passed the test and did not set a code.

The repair to the secondary air system fault was to replace the catalytic converters. Once we installed the new catalytic converters, we ran through the test plans and set all the monitors.

I had never seen a secondary air system fail this way. This car was a challenge. It is difficult when you have a test plan and you have the data, but it leads you to a dead end. It makes me remember that I’m always learning.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960-6988, email sfcsmv@gmail.com or visit sunnyvaleforeigncar.com.