The P0090 code indicates an issue with the fuel pressure regulator control circuit, which is crucial for managing the fuel supply to the engine. When this code is present, it often results in symptoms such as an unusual smell of petrol, especially at idle, and increased fuel consumption. The fact that the vehicle appears to operate normally at higher speeds but exhibits problems at idle and low speeds suggests that the fuel pressure regulator or its control circuit is intermittently failing or not operating within the manufacturer's specified parameters. This intermittent nature could explain why the error code sometimes clears itself, as the condition may temporarily improve or fall below the threshold for setting the code.

The likely root cause is a malfunctioning fuel pressure regulator or an issue within its control circuit, such as damaged wiring or poor connections. The fuel pressure regulator's role is to maintain the correct fuel pressure to the engine. When it fails, it can either cause too much or too little fuel to be delivered to the engine, leading to inefficiency, increased emissions, and the smell of fuel due to unburnt petrol. If not addressed, this issue can lead to more significant engine problems, including damaged catalytic converters, poor performance, and potentially even engine failure due to improper fuel-to-air ratios causing internal damage.