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I'm getting error codes relating to the secondary air injection system.

2 Faults Found:
17831 - Secondary Air Injection System; Bank 1
P1423 - 002 - Insufficient Flow - MIL ON
17819 - Secondary Air Injection System; Bank 2
P1411 - 002 - Insufficient Flow - MIL ON
Readiness: 0000 0000

I've replaced the Pierburg pump with a secondhand one and more recently a new one purchased from RockAuto in the States.

What else could it be? Vacuum pipes. The Combi-valves? Anything else? - dodgy lambda sensor?

Hi Andrew

Hmmm so you have changed the SAI pump for a new one, so in this case it's probably not full of water
It's strange to have this error on both, so it's something common to both combi-valves. I would be checking the vacuum lines that run from and to the secondary air injection solenoid (N112). Not sure where this photo is from, but maybe it was created by Ben? It shows the location of the solenoid. The other possibility is some blockage from the SAI pump to the combi-valves, but sounds unlikely when I say it out-loud

If worst comes to the worst, rip it all off, replace with blanking plates and decode from ECU. This is what I did... Purists would disagree

Regards
Craig Barlow

The SAI is more trouble than it's worth. I had a brand new one and it had drowned that within a short time. Code it out. Blank all the stuff off. Make sure any coding doesn't over fuel as that risks the life of the Catalytic converters.

TOPMAN wrote: ↑

Wed Sep 17, 2025 6:55 am

Hmmm so you have changed the SAI pump for a new one, so in this case it's probably not full of water

Yup, that's the usual culprit.

TOPMAN wrote: ↑

Wed Sep 17, 2025 6:55 am

I would be checking the vacuum lines that run from and to the secondary air injection solenoid (N112). Not sure where this photo is from, but maybe it was created by Ben? It shows the location of the solenoid. The other possibility is some blockage from the SAI pump to the combi-valves, but sounds unlikely when I say it out-loud

Yeah, that's my engine.

Definitely sounds like a massive leak. The solenoid valves can also leak or fail. Pull it off and blow into it while doing output tests with the Vagcom. See if it switches. Or just replace it. They are cheap.

TOPMAN wrote: ↑

Wed Sep 17, 2025 6:55 am

If worst comes to the worst, rip it all off, replace with blanking plates and decode from ECU. This is what I did... Purists would disagree

Yeah, this is a solution, although technically an MOT failure if anyone notices.

The combiner valves fail, too. Could be letting air escape. Maybe blow down each pipe to see if air passes or not.