P0521 on Toyota: causes and affected engines

2.0/2.2 D-4D (1 fault), 1.2 PureTech (1 fault), 1.33 Dual VVT-i (1 fault), 1.4, 1.6 and 1.8 VVT-i (1 fault) and 2.0 and 2.4 VVT-i (1 fault). Each fault has its own page with the affected years and versions, the symptoms and the repair cost.

Engine oil pressure sensor or switch circuit: range or performance outside what is expected. The control unit compares the signal from the oil pressure sensor (or switch) with the one it expects given how the engine is running. TaT describes the P0521 as a plausibility fault: the measured value is within the electrical range, but it is not consistent with what is expected. Autodoc Club says the signal falls outside the expected parameters or does not match the real engine conditions, and CarParts.com says the reading is higher or lower than anticipated. Modern cars use sensors that give a variable voltage depending on the pressure (not a simple on/off switch), and that information is also used to manage, for example, variable valve timing or a variable-displacement oil pump. It is a generic OBD-II code: it means the same on a Toyota as on any other make.^[1][2][3][6]

As soon as possible. The P0521 may be just a faulty sensor, but the code cannot tell you, and genuinely low oil pressure can destroy the engine: AutoDTCs speaks of bearing damage within minutes. That is why the sources that give an opinion advise against continuing to drive until you know: Autodoc Club says you should not keep driving; TroubleCodes.net, that the car should not be driven until the cause is found and that you must stop immediately if the engine warning light flashes or an abnormal noise appears; AutoDTCs, that you should not drive until the pressure has been confirmed with a gauge; CarParts.com, that if the oil and engine warning lights come on together you must stop and call for recovery; and RACE, that with the red light you must stop the car as soon as possible and switch off the engine, because of the risk of seizure. The sources are not entirely unanimous: Autodoc Club, for the P0523 (high signal), allows short trips, with a quick repair and stopping immediately if there are noises, and AutoDTCs points out that if only the sensor is failing, the car shows no other symptoms. Until the actual pressure has been measured, treat it as if it were genuinely low.^{[1][4][5][6][8][9]}