Any help would be greatly appreciated on any one of these 3 questions. I have an 05’ Sunfire with the 2.2 Ecotech. It has almost 90k on it and was fine until about 80k. I have been hit with 3 problems since hitting 80k. (1). Had to replace ignition module 4 times in last year, last one was about 6 months ago, GM tech advised to clean the grounds to the head and block, so far its been OK, but the problem really only happened when the ambient temperate drops below 40F. I am just getting into winter here in the north Midwest…..so we will see what happens. (Anyone had this problem?)…….(2) Within the last 60days the odometer digital LEDS are erratic in operation, they appear to be half on-half off…no pattern to which numbers come on or off….(Any ideas)……………(3) Have “CEL” P0420 that is persistent, have changed the Cat and front O2 sensor…then it was off for about 300 miles and came back on. I can clear it and it may stay off for 100-500 miles but eventually come on—same code. ..(Is the rear O2 sensor the problem here? Are aftermarket sensors a waste of $$ and if I need the rear one, should I just replace the front and rear as a set?)…this last problem is a royal pain as I have major $$$ in to this repair and the CEL is still on. The engine runs fine and there are zero drivability issues, but my local emissions check is about due and I am sure it wont pass………Also is there anyplace to get a GM service manual on CD? And/Or wiring diagrams?
Thanks for any help….

Let's start with the P0420. You've changed the cat (probably for nothing except to line the pocket of some dumb ass gas station mechanic) and the front O2 sensor. For now I would recommend changing the rear O2 sensor and the engine temp sensor.

I change my O2 sensors every other spark plug change or 100K (but I put 40K a year on my cars). The O2 manufacturers recommend replacement every time you change your plugs so that interval is up to you. Since you changed the front, might as well change the rear. As for the temp sensor, if the engine is getting too cold a reading from the temp sensor, it could be richening up the mixtures to the point of cat failure or O2 sensor failure. If the upstream O2 sensor is failing because it's getting fouled with too much raw fuel, then the signal may start to resemble the signal from the downstream O2 sensor (you'd need a graphing scanner or scope to see this accurately) and subsequently the ECU would set a P0420 code. Of course after changing the downstream O2 and the temp sensor, clear the codes. I routinely run the two KOEO and KOER tests after clearing the codes just to doublecheck that I didn't touch anything I wasn't supposed to. We'd need to take it from there if the codes sets again but it's a start. A leaky injector or intake or exhaust manifold leak could contribute to this problem but don't jump the gun yet.

With respect to the odo, a new cluster may be in order. I don't believe there's anyone who repairs these and if you go to a dealer they may want the old one as a core.

I can't help with the module as my '05 has served me faithfully these last 140K (knock on something).

As for the CD, I was able to find one on ebay a few years ago but it was a hacked data section from the GM techline program rigged to only recognize my '05 Cavvy. I've tried a number of times to copy it to no avail. I'd keep an eye on ebay and use the search line on this site. Maybe someone has access to a good copy. Helminc.com has the paper version for $135 + shipping.