58MVP040F114 w/infinity controller and 38TDB024 outdoor unit

This system was installed new in March 2004, and has had a persistent code 42 ever since. By persistent, I mean it throws a code 42 pretty much every time the heat comes on. The heat works properly otherwise, and the inducer motor does come on.

My installer has been very apprehensive about troubleshooting this thing from the start, and was pretty fond of just clearing the code to see if it came back, which it always did. He was out here again just today, which happens to be the last day of my 1-year p&l warranty, and he did manage to look a little deeper this time, but it has become fairly obvious that he has absolutely no idea how to troubleshoot the problem. I asked him why he doesn't just use the troubleshooting flowchart as found in the troubleshooting guide, to which he responded that the guide wouldn't apply to my system since it uses the infinity controller rather than a standard thermostat. So, he ended up disconnecting the combustion air inlet from the PVC so that it's now drawing crawl space air - the idea being to check for an inlet air restriction - and told me to call him if the code reappeared. It did, about an hour later.

I guess my questions at this point are, does anyone know where I can find the proper troubleshooting guide for the 58MVP/Infinity Controller combination that I can give to him? I know it's ridiculous, but if I hand it to him, he can't say it doesn't exist. I currently have the 58MVP-10sm guide.

The other question, has anyone run into a recurring code 42 on the 58MVP? If so, are there any common failures related to this code that he needs to be looking for, particularly if he refuses to follow established troubleshooting routines?