2000 bonne se, giving me codes P0401 and P0420.
I have checked the EGR passage, replaced the EGR, cleaned the throttle body, inspected and fixed a couple vacuum leaks, replaced the fuel filter, did plugs and wires late last year, and the MIL is on almost constantly.
Even thought about replacing the MAP sensor after reading an article, but it is progressively getting harder to start and not throwing a MAP code, taking longer to crank before firing. Usually worse when close to closed loop temp rather than stone cold.
This, along with it needing front brakes, rear warped rotors, the blower motor making noise, valve cover leaks, A/C bleeding off during a season with no leaks I can find even with dye, and a recent problem appearing with door locks, trunk release, turn signals misbehaving, and "check body" light coming on whenever it wants to, I am pulling my hair out.

Especially with electrical/electronic problems, you need to be very specific. "Problems appearing with" and "misbehaving" are far too vague to even consider a starting point. Some GM vehicles have a single body control module, however the Bonneville has several and the functions you are having problems with are spread across more than one. For example, the DIM (Dash Integration Module) and RIM (Rear Integration Module) both play a part in the trunk release. There is a module in each of the four doors, the DDM (Driver Door Module) acting as a gateway between the primary data bus in the car and the door modules for the right front and both rear doors. So, when you're talking about door lock problems, the issue can be a result of many possibilities. Some of the other modules include a MSM (Memory Seat Module), IPM (Instrument Panel Integration Module), TDM (Theft Deterrent Module), RCDLR (Remote Control Door Lock Receiver). I'm not sure what a "Check Body" light is? There is a MIL (Malfunction Indicator Lam) AKA "Check Engine Light", along with a "Service Vehicle Soon" lamp. The latter is the result of a fault detected by the PCM that is not emissions related. You will likely require a Tech2 in order to pull the codes related to this. Generic scanners will not pull many, and sometimes none of the codes, outside of the standardized OBD2 P0000-P0999 set depending on the brand and capabilities of the scanner.

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Bose Luxury Sound System w/Touch Screen Navigation, Addition of Factory XM, 2005 MY Antenna, OnStar Upgrade (3G),
RainSense Wipers, Backup Camera, '00 Style Door Panel Courtesy Lights, Heated Washer Solvent, 2X Remote Trunk Release,
Turn Signal Mirrors, Center Console Courtesy Lamp, Rear Outboard Heated Seats, PVD Chrome 18" Factory Rims, Upgraded
Carbon Fibre Appearance Interior Trim, Highly Modified Main Body Harness, Instrument Panel, Door, Door Panel & Headliner
Wiring Harnesses, Custom Fuse Box & Tire and Loading Information Decals, Additional Acoustic Insulation

Very informative. Thankyou.
Over the weekend, my left turn signals developed a problem that only seems to occur when the brake is pushed.
I'll try my scanner tomorrow and see what I can come up with.
OK, the turn signal problem seems to have vanished, However, now I have no rear defroster, the radio takes longer to shut off when parked and opening a door, and the blower motor will come back on by itself if car is turned off with blower running but will stop if I push the fan control button/dial.
I have codes as follows.
u1255, u1064, u1065, u1066-
u1300-
u1160-
From what I've read online so far, I'm thinking I should start by checking the wiring to the driver's door.
Any suggestions would be appreciated.