This has happened consistently over the past 4 years and makes no sense whatsoever.


The last time I had this was at the beginning of Sept. 2015 and since then not one occurrence whatsoever until today. This is a repeat of the same thing that has happen in the approximate same time period for 3 years and would occur one in a great while as much as 5 years ago, but always under the same weather conditions.

This occurs only when the temperature gets about 97 or 98 and have been driving a distance (but not necessarily a great distance) and would be sitting at a stop light for a period of time and then somewhere shortly after that when sitting at another stop light or sign and just pulling away, I will get a check engine light which will show a P1647/P1646. I have been closely monitoring it this year to see if I ever had any pending codes and not any until today. Sometimes I get a 1646 and it never gets to the 1647 to test or I get a 1647 with a completed test what would be the 1646. Sometimes I will just see pending, if I am checking. The car cools down, restarts and no pending any more.

I can let the car cool down a bit, restart and then drive a couple of places and restart 2 or 3 times and CEL goes away. If I just wait until the morning and the car cools down completely, it will go away when starting. It will then come back periodically when the weather temps are up and then the CEL goes away under the same conditions.

I know what the codes are about and the fact the system will try to run at test at various times, despite the fact that the O2 sensors are not even making use of the heated part when at idle in traffic and only really uses that in morning starts if the temp are not where they should be.

This has happened with the original O2 sensors and new ones from 18 months ago. It happened with the original cats as well as replacement cats.

The freeze frame of the data show nothing out of whack on anything when the codes hit. I have no other codes whatsoever that occur with these or any other codes at all other than P1111

I have some understanding that the 02 sensors work in conjunction with either the Temperature Sender / Sensor or Air Intake / Charge Temperature Sensor, but have never had any codes from those.

My own hypotheses is that due to heat and when the car sits at a stop that heat in the area of the 02 sensors get quite hot with the cats and such, that I am somehow getting a false-positive and the heated part of sensor does not get the test competed.

I would appreciate any ideas, explanations or suggestions. Perhaps an answer to the riddle might help.

Thanks

Tom in Dallas
05 S-Type 3.0 91k

 

Tom, I would suspect you have something going on with a wiring connection or the relay for the upstream O2 sensors. Anything that interrupts the sensor signal OR the heater circuit will set a P1646/P1647. Connections can be influenced by excessive heat, expansion etc. The O2 sensor heater relay, for instance' is in the front relay/fuse box in the engine compartment. The relay contacts, relay terminals, fusebox connector terminals, or the sensor connector terminals themselves could be giving trouble in very high temps. A new relay and/or accessing the wiring terminals and cleaning & applying a conduction stabilant may be your best options to try and prove that out.

Good luck!

 

That s the best and most sensible response I have heard. Since it seems to be happening on both sides, would you perhaps think it is something more common than an individual wiring of the sensors?

When you speak of the sensor connector terminals are you speaking of the connection of the o2 sensor first connection to the system and then does that wiring run to the relay directly or is there a stop between?

Thansk

Tom

 

It's shown in the electrical guide I reckon. You may need to look closely at the pages explaining how various things (splices etc) are shown.