Toyota Corolla 2001 engine warning light on?

Darren W

I have a Toyota Corolla 2001 1.6 VVTi. The engine warning light is on. I have had it code read and have P0420 'Cataylic converter system, Bank 1 - efficiency below threshold. The Cat and the downstream oxygen sensor have both been replaced but the downstream sensor is still giving low readings (100-200 millivolts). The upstream sensor seems to be reading correctly (switching between 90 and 800 millivolts).















Any advise would be greatly recieved.

M M M

Are they genuine toyota parts that have been fitted?, seen this before with non genuine cats fitted, could be a faulty oxygen sensor you bought, possible ecu problem, you need an auto electrician who knows what hes doing to do the various electrical tests on the car, good luck

Michael

P0420 makes me crazy. It seems to blame the catalytic converter (Catalyst efficiency below threshold [bank 1]) but what it really means is the O2 sensor in back of the converter is reporting fluctuation in the oxygen levels coming out of the converter. That is much more often the result of problems ahead of the converter feeding it indigestible exhaust.































One of the more common causes is a bad forward O2 sensor - it becomes sluggish (normal with age) and starts feeding the converter longer rich/lean pulses. Replacing the converter will make the code go away for a while because the new converter is slightly better than the old one, but then the code returns. You seem to have a different reaction to the converter replacement - the system still isn't happy.































The O2 sensor readings really tell a lot. The front readings say the sensor is still healthy (800 mv is still strong), suggesting the response is not sluggish either. In addition, the transitions say it is doing a good job of controlling the mixture. All should be well coming out of the exhaust manifold. The downstream sensor is saying there is too much free oxygen in the exhaust coming from the converter. Since you have already replaced the sensor I suggest looking for an exhaust leak between the upstream sensor and the downstream sensor. Whatever this is it is pretty unusual.