Toyota yaris 2001 Y Reg PO171 code engine light on

Question

I had a long drive in 30 degree heat and left car standing in direct sun for the weekend. On return journey, engine light came on for the first time ever. Could it have been the very dusty farm track or heat that has caused a problem? Someone took light off with one of those code readers and car was fine for a few weeks then the next long journey it came on again. Had air filter cleaned. Reader says not the 2 oxygen sensors. Garage said maybe dodgy fuel. Hoped air filter would solve it as it was very dirty but it came on again at 50 mph. So I've filled up with Shell's most expensive unleaded. Should I try some fuel additive too? Any recommendations for next step? Will Toyota diagnose it in more detail for free do you know? Thanks in advance for any help.

Answer 1

P0171 Fuel trim lean Bank 1

On the Yaris there is not much vacuum hose to cause the engine to lean, If the 02 sensor are working properly this would have to do with the fuel filter. It very advisable that you use a fuel injection cleaner,