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Secondary Air DTC Inconsistency

1775 Views 23 Replies 6 Participants Last post by  John_E
Background: 2001 Passat, AUG 1.8T engine
CEL came on, scan showed "incorrect secondary air flow"
I replaced the combi valve, which looked pretty gummed up, cleared all DTCs, and drove it
All readiness codes except secondary air cleared after only a couple of trips, but that one remained set
CEL remained off

Re-scanned for DTCs:
1. VCDS in engine control module mode or autoscan mode: "incorrect secondary air flow"
2. cheapo generic VW/Audi scanner says no codes, as does VCDS in OBDII mode

1. Should I go ahead and keep driving and hope the secondary air readiness self-clears?
2. should I clear all DTCs and start over? The last time I did this, I got into the described ambiguous state for secondary air
3. Why the secondary air DTC disparity?

Other stuff I should check? The relay/solenoid end of the control hose of the combi valve silicone hose had hardened and pulled off easily when I was doing some vacuum tests. Since just the very end, overlapping the nipple, had hardened, I wonder whether I had a small leak there. That, coupled with a sludged-up combi valve, might have caused my CEL. I have since cut off the hardened end of the hose, so if there was a vacuum leak there, that should be remedied by now.

Other suggestions? This thing is driving me nuts, and I need to pass smog within the next 3 weeks.
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Thank you. All true.
Replacing the combi valve made a huge difference, in the sense that I no longer got a fresh 0411 CEL soon after clearing the old one.
I did also replace the SAIP's outlet flex hose. The inlet flex hose should be irrelevant, unless blocked.
Having read that an old ("slow") pre-cat O2 sensor could prevent the SAI system monitor from clearing, I did replace it, as well.
At this point I am thinking either the combi valve is still not opening all the way or that the pump is not putting out as much air volume/pressure as it is supposed to. Everything in the SAI system was 22 years old, original with the car, except for the 3mm control hose for the combi valve, which I replaced with silicone a good 15 years ago, because the original one had cracked just past the combi valve nipple.
At this point I am down to the pump and/or the combi valve's control solenoid. Relay and fuse were the first things I ruled out, because the pump does run.

To me, the weird part is still why does the VCDS report "no faults" in generic OBDII mode, as my cheapo off-the-shelf scanner does, but "secondary air insufficient flow" in either autoscan or engine module reading modes? Why no CEL? Is the VCDS code somehow specious or otherwise misleading?
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Thank you -- that makes sense. I cut the hardened end off of the solenoid-to-combi line, cleared the DTCs, and so far, so good. I may go ahead and replace the rubber F hose, given its age.
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I did find an answer to the "pending DTC" question on

"The PCM will successfully complete the secondary AIR system monitor if the proper voltage signal is detected from the upstream oxygen sensor. If an incorrect oxygen sensor signal is detected the PCM will set a pending code.[1] The PCM will command the check engine light to illuminate and store a DTC[2] if the fault occurs during 2 drive cycles." So, I conclude that my SAI self-tests have been intermittently passing and failing after I replaced the carboned-up combi valve. Before I replaced it, they had been more consistently failing. I'll see if my combi valve vacuum control hose fix completes the repair. I have ordered an F hose, per your suggestion, as a backup, since it is relatively cheap and will definitely need replacement at some point, anyway.

[1] detected by VCDS in scan mode, but not in OBDII mode
[2] detected by cheapo scanner and VCDS in OBDII mode
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Update: No more CELs, DTCs, pending or otherwise, but the VCDS test of channel 77 (SAI) aborted after two test on/off cycles. I also got -100% in the third window -- does this mean complete absence of SAI airflow at the O2 sensor? Interestingly, replacing the carbon-clogged combi valve did make a significant difference, because before I couldn't drive a round trip without triggering a CEL. I'll get back on it Thursday to check vacuum control to the combi and air output of the pump. I have a replacement solenoid valve available, if so indicated. Thank you, as well, for the vac reservoir hose suggestion -- I'll give that a close inspection, or see if it is among the hoses I replaced several years ago with silicone.
Update: applying vacuum to the combi valve's control nipple and jumpering across the SAI relay does produce air flow at the tailpipe. Cold startup does cause the pump motor to run.

I need to retest with a proper seal on my vacuum gauge, but I do not think I am getting suction from the combi valve's control solenoid under the intake manifold. If this guy is indeed bad, any hints on how to remove and replace it? It is pretty inaccessible down there, even with the throttle body air inlet duct and the coolant overflow tank out of the way. Maybe drop the panel that holds the two solenoid valves under the intake manifold?
I'll give that a shot. Yes, the Bentley book shows a triangular plate with a mounting bolt on each corner.

I was a bit slow on figuring this out, but here is almost undoubtedly what happened, based on the discolored vacuum line with the split end and series of CELs and failure to set monitor:

The combi valve got carboned / clogged up over 22 years and did not fully close, leaking exhaust gas back into the control line to the AIR solenoid switch. I was eventually getting CELs because the combi valve would not fully open or close. Meanwhile, the exhaust backflow gradually carboned up the valve portion of the solenoid switch. Now, with the new combi valve, no more CELs, but also not enough vacuum to pull the combi valve open to set the SAI readiness monitor.

What I find interesting is that the SAI solenoid is driven by one pair (green/yellow and yellow/green) of wires from the ECU, whereas the secondary air pump relay coil is driven by a different set. I would have expected (and perhaps this is true) the pump and the solenoid to be energized together, but perhaps the intent is that the pump start before the valve opens and stop after the valve closes, to protect against the sort of backflow I was seeing with my defective combi valve.

Anyway, if I can't somehow clean out my control valve without damaging the plastic (I am going to try applying suction from its outlet to the combi valve), then my only recourse is to dive in and replace the solenoid. Thank you for all of your assistance -- this one has been a bit of a puzzler, and different from the usual leaky vacuum lines, bad pumps, and other common problems cited by others.
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Not if the combi valve diaphragm has a leak in it. That is the only way I can explain the heavily discolored 3mm vacuum line with the hardened and split end at the solenoid control valve. I'll check the old combi valve more closely to prove or disprove this theory, which is the only one I can come up with that is consistent with the observations.

I have no vacuum suction from the control solenoid, and the direct combi valve suction test works, allowing blower air through the exhaust pipe, so if my theory is incorrect, I am out of aces at this point. With the old combi valve I was getting a fresh DTC within two trips, often just one, after clearing all DTCs. At this point, no more pending or full DTCs, but absolutely no luck setting the readiness monitor, and my VCDS reports -100% for secondary air flow when I try to force the unit 77 test, which aborts after two attempts.

F hose looks great. Other stuff I should be checking? Brand-new combi valve and brand-new Bosch O2 sensor.
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Latest update -- I had a close look at the original combi valve, and its control inlet nozzle is indeed carbon-coated, further supporting my theory that there is a leak in the control diaphragm, which would account for that, the discolored 3mm silicone tubing, and the (evidently) clogged solenoid control valve. I am hoping I can clean out and rehab the control valve, to avoid removing and replacing it. I still haven't worked out how I would cut the Oetiker cable clamp.
Can't get my dremel, or even a drill bit, in there.
New info: the old combi does hold vacuum at the control line, which blows my theory that it contaminated the solenoid control valve. I am wondering if I have a bad check valve that is robbing my solenoid control valve of a vacuum source. Still can't explain the discoloration of the control solenoid valve - to - combi valve silicone hose or the carbon buildup inside the control nipple of the combi valve. Boost leak through the vacuum lines??? I think checking the check valves is my next move; open to other suggestions, as well.
Thank you, Andreas.
My combi valve was pretty gunked up, and it is still interesting that I threw secondary air CELs with it, which tells me it wasn't opening and closing properly.
With the new combi valve, no CELs, but no secondary air readiness, either.
Still working off the observation of no vacuum coming out of the control solenoid valve, I'll go ahead and replace the braided rubber hose that connects to the tail end of the F hose, because it is cheap and accessible under the coolant recovery tank. Does anyone know the ID? 6mm/1/4", perhaps??
OK, I got everything out from under the intake manifold -- not too bad once you get the feel for it. I pulled the old relay-solenoid, and it won't hold vacuum, unlike the new replacement unit I bought. I have been replacing various original vacuum lines and related rubber parts as long as I am in there, but I am confident that my CEL and failure to set secondary air monitor were attributable to the combi valve (which admittedly probably is cleanable) and particularly to its control solenoid.

If anyone needs any AUG 1.8T tips on the disassembly procedure, fire away. I'll be glad to help any time, now that I am a veteran of this. I'll be a couple of weeks late with my registration renewal, but I did pay the fee a month ago to avoid any potential late penalty.
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