I have a Toyota Yaris 1.3 VVT-i with 2SZ-FE engine. The car has 100K of mileage (I recently bought it with no service history). Most importantly, its check-engine light is on for "catalytic converter below threshold", which has also appeared in the MOT for the past 2-3 years.

I looked for Engine oils for this car, and based on suggestions on different websites, Castrol Magnet A5 and/or A5/B5 is the best choice. However, I've read in another post that the choice of oil (C3 vs A5, etc) should be made based on whether the car has a Catalytic converter.

My car does have a catalytic converter, but it is performing poorly, and I don't want to spend hundreds of ££ to replace it on such an old car. The question is: does picking a specific engine oil help improving the Catalytic converter performance for the MOT test?

Also, what engine oil is best for my car in its current situation?


No, using a “magic” oil won’t repair the cat.

But using the incorrect oil can easily damage it.

Replacing the cat will be needed to get the car road legal.

  • "Check engine light comes on at engine start" (for any reason) is only a failure item for cars first used after 2003. Otherwise, if it passes the emissions test (even with a defective cat) it is legal. Note, this rule change was made in 2018, so presumably the OP has an older car if it passed in 2019 and 2020.with the light on.
    – alephzero
    Apr 25 '21 at 18:36
  • @alephzero The engine light turns on for catalytic converter, but then each time it appears I turn it off by the ODB2 tool. The one who sold me the car has probably done the same because the light wasn't on when I bought it (then it appeared a few days later). Do you mean that if that light is on when doing the MOT, then it fails the test? Would turning it off by the ODB2 tool prevent this?
    – Ali
    Apr 25 '21 at 23:16

Catalytic converter performs best when the exhaust gas temperature is between 300-400degree celsius. Engine oil, a part from providing lubrication to the piston-cylinder also reduces the cylinder temperature thus reducing the exhaust gas temperature slightly. But the change is not very significant. we can use specific engine oil to improve performance of engine but the catalytic convertor will not get affected much.

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