code 31: Park/Neutral switch
code 38: Brake input circuit
code 43: Electronic spark control circiut or Knock sensor problem

can I do any of these myself
and are any of these codes detrimental to the health of my car.

Please add year and model to your signature so we can help you better.

[edit: Is your car starting, shifting, and driving normally? If so, perhaps the repairs were made and the codes were never cleared from the computer. If these codes were active, seems like you would be having some serious issues like no 4th gear, no TC lockup, and 10 degrees of retarded timing yeilding sluggish acceleration. To clear codes, disconnect the negative battery cable for a couple of minutes, then re-connect it. Drive the car for a while then check for the codes again.]

Most of the following information was lifted from the '92 FSM. Also included is some guesswork by me. I only have personal experience working with the PRNDL switch alignment and knock sensor circuit. Others more knowledgable with the 31 and 38 codes will hopefully chime in.

Code 31 PRNDL switch circuit:

The PRNDL switch is mounted on the transaxle assembly. It is possible that the switch has loosened and become misaligned. There is a procedure for re-aligning the switch.

"When code 31 is present, the PCM assumes the D4 (overdrive) position until a correct combination is received by the PCM. Therefore, some gear select positions may not be possible until the fault is repaired."

Check for poor connections, rubbed through wire insulation, or a wire broken inside the insulation on the wires from the switch on the transaxle to the PCM.

Inspect the PCM harness connectors for backed-out terminals, improper mating, broken locks, damaged terminals, poor connections and damaged harness

Functions are tested with a Tech 1 scan tool at the dealer.

Code 38 TCC Brake input circuit: (tcc= torque converter clutch)
This switch is attached under the instrument panel and presses against the brake pedal arm so that when you depress the brake, the switch cycles. Maybe it is just misaligned, or not properly adjusted.

"A single code 38 is the result of an open in CKT 420 between the splice and the PCM, poor connection at PCM, or possibly a faulty PCM."

"If the brake switch has failed in an open state, TCC will not engage."

"When code 38 is set, the PCM will not allow 4th gear or TCC operation."

Need year and model specific info for further tests

Code 43 knock sensor (ESC circuit): (paraphrased)

"If the knock sensor circuit becomes open or shorted to ground, code 43 will be stored"

"If Code 43 is set, PCM will retard spark by 10 degrees"

Check the knock sensor connection on the front of the engine near the starter.

I apologize
1992 bonneville sse
163,000 miles
new tstat

Based partially on your year:

1: I assume you have the Magnavox ignition.....square coils? All one unit instead of 3 distinctly different coils mounted to the ICM? Upgrade to the Delco2 ignition or seriously consider it. Either your crank sensor or ICM is the likely culprit of the code 43.

2: Code 31. If you aren't having starting problems, ignore it for now. We'll come back to that later.

3. Code 38. This concerns me. I'd like to know the state of your brake lights for starters.

My rear brake light does not work. could that be a reason for one of the codes?
Is this a major concern?

ALL of your rear brake lights? Yup. That'* a concern, and if they're all out, that might be related to the Brake Input Circuit.

no it'* just the main break light located above the trunk.

Unrelated, but change that bulb.

where is the brake input circuit?