Lexus NX Throttle Valve Defect

I had a similar problem with my Lexus NX. The symptoms you described point towards a throttle valve issue, possibly related to the accelerator pedal module. This module is responsible for communicating the driver's input on the accelerator pedal to the engine control unit (ECU), which then controls the throttle valve position, regulating the air intake and consequently fuel injection. Defective values from the accelerator pedal module could result in improper air-fuel mixture, hence the poor fuel efficiency and starting issues. Did you try checking for any error codes with an OBD scanner?

An OBD scanner...right. I haven't tried that, to be honest. I'm not super techy when it comes to cars. I know where the gas goes, and that's about it. So, if the accelerator pedal module is sending bad info, is that something a regular mechanic can diagnose, or am I looking at needing a specialist?

A mechanic should be able to diagnose the issue using diagnostic tools to read sensor values and perform tests on the accelerator pedal module and throttle valve. Given the symptoms and the potential cause, a visit to a trusted repair shop is advisable for a thorough inspection.

Took your advice and went to a workshop. Turns out, the throttle valve was indeed defective, and the accelerator pedal module was the culprit! New parts, a bit of labor, and 205€ later, she's running like new! Thanks for pointing me in the right direction!