My CEL keeps coming on the exactly same spot to work every morning, accelerating up a hill in 3rd/4th gear.
It also comes on sometimes in higher speed zones/longer trips. What might be causing it?
 
For reference the car seems to be operating fine. I can't check for codes as it brings up no errors as the CEL goes away when I turn the motor off (ignition still on).

To check for error codes you need to bridge E1 and TE1 in the diagnostics box.

just_ace
To check for error codes you need to bridge E1 and TE1 in the diagnostics box.

I know, but Can you do this with the motor still running?

You dont do this with the engine running.

The ECU will store the error codes, so you can turn it off, bridge the terminals & retrieve the code.
 
You're not running a Power FC though are you?

Probably Code 51 - Switch Signal.
 
Check that your TPS is working and within spec.

Code is 42 (No Speed signal) This happens when no SPD signal has been received for more than 8 seconds with the car between 2 and 5k RPM (hence it happening in the same spot or highway driving).
 
I'm in progress of changing the speedo nylon gear so i'll clear the code once I replace it.
Good that it wasn't an actual engine issue. :)