Thread: Flashing check engine light Discovery 3S 4.0 V6

-25% is negative, it's pulling all the fuel out of that bank that it can, this means it's being measured as too rich by the O2 sensor, one cause for it being too rich would be a lack of air coming in, because it can't get the exhaust gases out.

Your cat is toast for sure.

Originally Posted by Cambo@oldjaguar

Your cat is toast for sure.

Hmmm, this is a well reasoned position. It is a little odd that there is no error code being generated for catalyst efficiency.

Interestingly it starts well on open loop, then as soon as it heats up and runs closed loop the STFT/fuel trim goes mega negative (rich) then reverts to open loop within 30 seconds- presumably due to the misfire. It is odd that there is no code for the running rich condition.

Looks like I need to find a cat lady.

Originally Posted by Julian B

Interestingly it starts well on open loop, then as soon as it heats up and runs closed loop the STFT/fuel trim goes mega negative (rich) then reverts to open loop within 30 seconds- presumably due to the misfire. It is odd that there is no code for the running rich condition.

This is a common symptom of a dirty oxygen sensor. Voltage that is applied to the heating element leaks into the sensor output showing up as extra rich. Moisture from combustion settles in the sensor for a short time making a conductive path with the dirt, until it heats up properly. The misfiring is actually from excessively lean mixtures. Try a new oxygen sensor before embarking on any major cat control programme.

Well to rule that out, swap the O2 sensors over to opposite banks, if the faults stay on that bank it's the cat, if they move to the other bank it's the sensor/s.

Shame they are such pigs of things to get to.
Pre or post cat sensor most likely?
If I had a little camera scope I could peek down into the cat!

Originally Posted by Julian B

Pre or post cat sensor most likely?

Pre cat of course, they control the mixture. Post cat just checks for correct cat operation.

Last year I had someone bring in a later model Hilux with pre and post cat sensors, his "mechanic" had told him he needed a new cat (because the codes said so). Turned out the plugs had not been changed since the ute was new (180K ago) and the resultant constant misfire just overloaded the cat. New plugs, cleared the codes and away it went.

Originally Posted by bee utey

Pre cat of course, they control the mixture. Post cat just checks for correct cat operation.

Last year I had someone bring in a later model Hilux with pre and post cat sensors, his "mechanic" had told him he needed a new cat (because the codes said so). Turned out the plugs had not been changed since the ute was new (180K ago) and the resultant constant misfire just overloaded the cat. New plugs, cleared the codes and away it went.

Hmmm. So the plugs were average but replacement hasn’t changed anything. It isn’t running poorly except a slight bumpy idle which suddenly appeared on a morning start along with the codes.

Ill change the pre cat sensors and see if it fixes the problem.

I reckon you'll be doing this next;