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Finally had a chance to devote another morning to this to no avail. (Somehow, I'm not getting email notifications when replies get posted so apologies for the delay in holding up my end of the conversation.) On the diode, I did a voltage drop test across the diode from both the leads of the diode and the A32 and A37 pins to which it is soldered. Got a voltage drop of ~580 mA in the correct direction and no flow in the incorrect direction. Checked another diode from the same lot with the same multimeter and got consistent results. In short, I don't think I fried the diode. I pulled the vacuum line from the solenoid valve with the engine running and there was no vacuum coming out of the line. I still left it disconnected just in case some residual vacuum pressure (or lack of pressure) in the line was causing the valve to open and creating the bigger voltage drop. The pump is still unplugged. I started the car, cleared the code (P0410 Secondary Air Injection is still the only code being thrown) and sat in the car at idle for a few minutes. No CEL. Turned the car off, turned the car over, and the CEL came back instantaneously. It's throwing the the P0410 code again and nothing else. The only other thing I can see that seems out of the ordinary is that the OBDII reader takes a freeze frame of the readings on all the modules that presumably is from when the fault was thrown and it indicates a CL-Fault on Fuel Sys 1. (I think that means a closed loop fault?) That doesn't seem to generate any codes in its own right and I suspect it is not related and perhaps even the correct setting immediately on starting the car, but I thought I would mention it in case it sheds any light. Very frustrating not to get this fix to work but I'm so glad you guys are all here to help out.

Finally up against the emissions inspection deadline. Completed the mod but am now still getting the P0410 code within a few minutes of clearing it. No other codes are being thrown. The diode is inside the ECM and in the right place and correct direction. I've unplugged all of the connections to the pump and I've put a blockage in the vacuum line between the tree and the solenoid, which is still plugged in. (I'm leaving things in place as I want it to look normal under the hood.) I've cleared the code a few times but it comes back pretty quickly. In terms of what I've done wrong, I had the wrong diode, a 1N4001 instead of a 1N4003, soldered in place for a few months (not sure how I misread that instruction in the first place but I've now replaced with the correct diode), I somehow also thought that I had to leave a "ground" wire attached to the pump so hadn't unplugged all of the wires from the pump. This also was the condition for over a month. So, I drove the car with that incorrect setup for a while and the CEL on the whole time. I finally cleaned up all the mistakes. Any idea whether having it done wrong for all that time might be causing my problem? Alternatively, any idea what else I might be doing wrong?

Thanks for sharing this Volvo5.0! It will save me a lot of headaches and quite a bit of pocket change!!