That is a big number and very unsafe, shut it down and fix or replace, Until its fixed cut the power to unit, pull the low voltage to gas valve and plug gas line There is nine ways to to get high CO off a furnace, with that big # it rust in heat exchanger and burner very dirty. low presser - high presser could be part of it, Burner off set can make big number or flame hitting wrong on steel. Most holes in heat exchangers start low and when blower turns on makes the CO rate get higher and higher 1st reading will be like 200 PPM then just get higher and higher could be 1000's at the end of a hour burn with burner getting air blow-en on flame. If the hole is very low and not hitting flame - some times will not change CO number. I had a boiler last spring with 3000 PPM CO cast iron cleaned it and now its 22 PPM after rising gas presser from 1.73 in water to 4.02 inch water Column. code date is 1970 June 5 burner If I read it right had a fan relay for AC but blower motor was just 1/5 hp so must put in larger HP motor for AC.
Did you observe the flame as the air handler started up? Did it remain stable, or change pattern?
The production of carbon monoxide is a byproduct of incomplete combustion and there are numerous possible causes. If you are reading this in the vent (which I assume) it may not necessarily be a leaking heat exchanger (of course it could be). If your reading it in the airstream, it is still possible that the culprit is not the furnace. It is possible to have significant CO production with intact heat exchangers and its possible to have a large heat exchanger opening without high CO readings. That's the dangerous part about relying only on CO readings - too easy to miss and too easy to misdiagnose.
There are a lot of variables, but with any combustion issue the first three things to look at are: gas pressure, combustion air, and venting. Before changing the equipment it is important to know what is causing the currant problem. If it is an installation/application problem you could easily pass on the problems to a brand new furnace.
By the way, is this furnace on natural gas?
I would be happy to provide further info if you like, but it will require further investigation.
There is a lot of data that you could collect that should lead to the answer. However if this is not an area where you feel comfortable you may want to get a "qualified" HVAC tech to go through it with you. Beware, as you probably know not all HVAC guys are "qualified".
You really need a gas certified technician who really knows what they are doing, There are many possible causes of high CO in the flue: low gas pressure (failed pressure regulator; sticking gas valve not opening all the way; kinked soft copper supply line; undersized gas piping (especially when additional gas appliances have been added to system); rusted out burner tube(s) (or gun); blocked air vent or venturi; etc.
A cracked heat exchanger is not going to be a cause of such high CO.
It is my understanding that keeping your probe running in such a high CO stream will quickly consume your CO sensor. My Bacharach Fyrite Insite will not read above 2,000 PPM (but my older Fyrite Pro 125 will read higher). When I see the reading going above mid-teens, I pull the probe. I don't need to know exactly how ridiculously high the CO is, I just need to know that it is ridiculously high!
If you are not gas certified, you may not have a legal right to disable a furnace, even though you could try to argue the "do no harm" mantra. If the client will not voluntarily allow you to disable the furnace, then call the gas supplier and report your concerns, and document.
And then we get into the discussion again about reporting "air-free" readings. There are many other posts on this subject. Just report the CO level found in undiluted flue gas.
A cracked heat exhanger would not cause high CO (unless it affected the operation of the flame).
A cracked heat exhanger might allow the CO into the house, did you check ambient & a supply register for CO?
High CO can mean a burner needs cleaning, the air/flue mix needs adjusting, gas pressure is off.
But does it matter?
It's such an old furnace that you can justify replacing it on several counts, especially safety.
I have seen 4000 PPM CO at the breech of the flue and only 335 PPM CO at top of brick chimney 55 feet up with lots of air/flue gas mixing. CO tends to go down the longer it goes down pipe or chimney. Still with over 100 PPM clean it up and test out. When I test out I look for 02 and temp of flue gas as much as CO. Is the CO getting higher? - that the most danger.
I have seen 236 PPM at top of AC coil and at suppy grill 152 PPM with a hole in heat exchnger blowing air on flame - I used the fire man tank and mask. If you do not test you do not know. I go though 3-5 rebuilds a year