1996 Buick Century V6 with recurring P1406 after EGR valve replacement
I'm having a bit of a troubleshooting problem on my car (96 Buick Century Custom V6 3100 with about 93k miles) and I'm hoping somebody here will have a suggestion. In the last couple of months I've done quite a bit of work on it, but it seems to have a recurrent gremlin that I can't figure out. In the last couple of months I've had the following done (mostly just before the extended warranty expired): engine replaced with a rebuilt one, fuel injectors, ignition control module and two bad ignition coils replaced, replaced the fuel filter, the IAT sensor, and finally most recently replaced the catalytic converter.
About a month ago, and before I replaced the catalytic converter, my check engine light came on with a P1406 (EGR Valve Pintle Position Sensor) code. When I removed the valve it had quite a bit of carbon buildup, so I went ahead and replaced it and the gasket. That seemed to resolve the problem - cleared the code and it didn't reset, nor did it have any major performance issues.
About a week ago the MIL came on again, this time with a P0402, so I took it to the shop and they tested the catalytic converter and let me know it was shot (which I expected), and they replaced the catalytic converter. A day or so later the MIL came on again, this time with the P1406 code again. I just replaced the EGR valve a month ago so I can't imagine that it'd be bad already, though I guess buying a bad part is always a possibility. At this point I'd love to get some idea of what my next diagnostic/troubleshooting step should be -- I'm out of warranty now, so I'm totally out of pocket if I need to take it to the shop for (pricey) diagnostics. If I can't figure out the problem and repair it myself I'd at least like to narrow it down a bit to try and save on the diagnostic costs.
I'd appreciate any thoughts or suggestions anyone has for where to go next on troubleshooting this.