Since July 2006 my 87 Crown Victoria failed state emission test three times for NOX limit 3.0 first 3.69 then 3.18 grams per mile (big deal 0.18gm...the car is driven about 3000 or 4000 miles a year and the state knows the miles driven but forget about arguing...)

Anyway, I am working on finding and fixing problems like worn out vacuum lines and loose electrical wires, connectors, test and clean EGR valve, etc.

I tried reading codes with a Scan Code Reader. Over two or three days checking, the computer codes were sent okay during several KOEO and KOER tests. Codes always same: #1, #11 (the ID/pass codes) #55 (I think) and #67 (Im certain).

But computer stopped sending codes after the LAST of two or three KOER tests.

I have Code Defs and Ford book (below). #67 is Neutral Drive Switch and Ford book says yes this #67 is a NO CODE condition meaning the NDS can prevent Codes from being sent.
I forget what #55 is and you are already patient reading I will leave that alone now.

I know NDS was circumvented by old Alarm install I am fixing that now. (I purchased the 1987 old boy used--wish I had got it when much younger this car is great with rear sway bar and hand-crank windows it reminds me of my two 1963 Ford XL convertibles--that 406 was mean sounding with solid lifters and factory headers...pretty fast too...only add-on was Holley 780cfm...but that was long time ago and the 87 out-handles and out performs those old ones.
Boy I wish the 406 had SEFI it wooda had 485hp!! but I never like driving fast just clean, almost factory stock originals).

So now I am trying to get my computer working again, I am waiting for the required 60-pin Breakout Box EEC-IV to go further. Yes the car runs normally and EGR and CANP are operating; Ford E/E Manual says this means computer okay. But I get No codes now. AND I tried the ANALOG VOM method of code reading that dont work either. As mentioned above I am cleaning wiring connector contacts on a car this old anything can be wrong

Helm 1987 Ford Engine/Emission Service Manual and Haynes Computer Code Book both say:
You need a DVOM with 10 Mega-Ohm Input Impedance.

My question is do ALL DVOMs HAVE 10 Mega-Ohm Input Impedance. I am not referring to any measurement scale on the face of the meter this specification relates to the DVOM internal circuit design and my little $20 DVOM (Mfgr year about 1990) booklet, other catalogues, etc. do not list the Input Impedance.

Do I have to spend another $100 or $200 on a better DVOM before I use the breakout box.