Published: 2022-01-22

MECHANIC: C-Elysée Citroën I have error code P0420 read from the engine module with KTS: "Catalyst efficiency verified - efficiency too low"...
EXPERT:What car is it?
MECHANIC:A 2014 C-Elysée.
EXPERT:Do you have a VIN number?
MECHANIC:Sure! Can I dictate?
MECHANIC:Victor Francis Seven Delta <the rest cut out>.
EXPERT:Valeo controller and P0420, the famous error.
MECHANIC:Oh! Interesting.
EXPERT:The end of the cat.
MECHANIC:The end of the cat?
EXPERT:Yep, the catalytic converter is done, although that’s quite early... Does it run on LPG?
MECHANIC:No, no, no, petrol, but here’s the question - clearly from the error you can see that the efficiency is poor, meaning that it's burnt out and not raising the temperature...
EXPERT:And have you checked? The first and second probes show almost the same thing, right? This error pops up when the second probe doesn’t confirm the catalytic converter is working well. So you can still have some hope that the second sensor is lying.
MECHANIC:Second, meaning not the first one from the engine, but the second one, right?
EXPERT:Behind the catalytic converter.
MECHANIC:Right... because to be honest, we looked at the visual condition of the catalytic converter and you couldn't see that it was worn out. Nice and clean, honeycomb in one piece...
EXPERT:You'd have to help it out by warming it up a bit... but it’s also worth checking this probe and what parameters it shows. So check the voltage values of the lambda probe - bank one, sensor two. The probe should, when you warm up the exhaust, rev it up to 2 or 3 thousand for about a minute to let the catalyst warm up a bit, this probe should show voltage above 0.5 V to 0.8-0.9 V. It should jump between 0.5 and 0.9 volts. And the first probe should oscillate in the range from 0.1 V to 0.9 V. So if you take two probes, the voltage of the first and second probe...
MECHANIC:In the actual values I'll choose two probes next to each other, exactly...
EXPERT:Yes. The second probe is supposed to have voltage values from 0.5 V upwards. And that indicates that the catalytic converter is working. So you would have to check what the voltage values are, because if this voltage on the second lambda probe goes below these 0.5 volts then the computer is just throwing this error P0420. And that's what happens when the catalytic converter doesn't work well, but it could also be that the probe is lying, there could be a voltage drop on the wires - we can't rule that out.
EXPERT:That's why I would check what they show, for example, when it's cold and the catalytic converter isn't working yet, both sensors should show practically the same thing. When the catalytic converter starts to heat up and the reactions begin, then the voltage of the second probe shouldn't drop to 0.1 V and must be definitely above 0.5 V.
EXPERT:So you have to observe this - if this catalytic converter is actually damaged then nothing will change, the first and second probes will show similarly, there will be no difference between them. Generally the second probe controls the operation of catalytic converter, so if signals on both probes are oscillating in a similar range then the catalytic converter doesn't work.
MECHANIC:Okay, okay, I'm gonna warm it up and take a look. Cool, thanks a lot, cheers.
EXPERT:Cheers, see you later.

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