Retrieve and post the diagnostic code(s) the next time the SES light comes on with the engine running.
The above is new to me. A mechanic buddy of mine, ASE certified, Level ? GM Mechanic, has subscription to some kind of GM software and online access to GM service info. (Works at American Yachts right now but pulled all this stuff up on his computer to show me).
He showed me to take a u-shaped paper clip, stick it into "B" and "A" terminals, to the key to ignition on and then showed me how the computer will talk to me with a series of flashing lights. Told me to write down the numbers and then I can use these to get the error codes.
But he showed me how to do it with the engine off, key on. You're telling me to do this with the engine running? You probably mean if I had the actual tool that you plug in to check codes, I just have the paper clip. 
No contradiction or confusion was intended. A fault code has to be stored before it can be retrieved. Viable faults generally occur with the engine running during day-to-day use. I intended to convey that you retrieve codes after the SES light has come ON during normal use.
Supplemental details:
The SES light illuminates as soon as ignition is energized,
before starting the engine. You will retrieve Code 12 (no tach signal) whenever the ignition is switched ON and the engine isn't running. Code 12 is a volatile fault that isn't stored in ECM memory, but provides an indication that the ECM self-diagnostic feature is functioning. The idea behind my post is, for diagnostic purposes, we are interested primarily in active codes that set due to actual faults, which result from an operating drivetrain. Once fault codes are stored they can be retrieved without starting the engine, using a paperclip (or a code reader), as your friend described.
Beyond simple code retrieval, a sophisticated scan tool such as your friend uses provides valuable diagnostic information on a running engine in near real time and can record difficult to diagnose transient problems while driving, for subsequent playback. OEM scan tools and software are the premiere diagnostic tools that aftermarket manufacturers try to emulate.
Does it have the O2 sensor?
I know O2 sensors are the norm for most cars but I don't know where to look?
There's a wire going to a fitting on the exhaust header, driver's side, I don't remember looking to see if there's one on the passenger side? I can go look.
Would this be the O2 sensor?...
Your '87 has one O
2 sensor located where you were looking on the driver's side.
Beautiful truck, by the way!
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