99 es 300. have had it for 6 years.
a year ago check engine light and trac off light came on. car would occasionally go in limp mode but then reset it self and drive normally.my mechanic changed a knock sensor, light went off and normal performance returned.
the same 2 lights have come on again recently. I have not noticed any change in performance this time. any one else experienced this scenario? should I ignore it or do something about it?

 

Read the DTC to see what is what.

 

DTC?
is that computer codes?

 

You can buy an inexpensive OBD2 reader online or have the the codes read at a lot of auto parts store. If you get those codes, we can help you diagnose them.

 

Diagnostic trouble code

 

got it checked at autozone yesterday.
error code: PO 171 system too lean bank 1
definition: code PO 171 indicates that bank 1 system is too lean for a predetermined period of time.
most likely solution: replace air/fuel ratio sensor (AFR)
car is running as well as even. averages 27/G
I hear occasional very light knocking only when my son is driving and is heavy on gas pedal. but I am also using 87 octane.
should I leave it aor do something?

 

Clear the code only. That code is the most common code that comes up with these engines and may have just been a temporary hiccup with the air/fuel ratio. Sometimes this comes from a dirty MAF sensor.

 

To save you another trip to Autozone, you can pick up a DTC reader pretty cheap on Amazon. I paid $35 for mine, works great.

Just be warned if you clear codes before a state inspection, they may still fail you. The codes have to stay cleared for a period of time before they fully clear, which should allow a pass.