Hi all,

I changed both upper O2 sensors (RH in 2018, LH last week).

First off, I now have a P1646 fault code, indicating that the rh sensor is toast again - but it's not, I tested and it gives a 170 ohms resistance, vs. 160 ohms for the newer, lh one -- bought from the same eBay vendor.
(I don't suppose this is an issue ?)

Perhaps a busted fuse then? Battery disconnected all night but no change.

Second thing is that the fuel trim values aren't normal. I'm getting a steady 85 and 81% for the upsteam sensors, and like 40% for the downstream ones.
One of them keeps varying, from -5 to +40%, the other remains fairly steady around +40.

I'm not sure what to make of it. It shows that the ECM is adding fuel permanently because too much air in the exhaust gases?
Does that simply imply there is an air inlet in the circuit after the mass flow sensor? (Brittle plastic and had to put tape around broken tubes, but it seems to be airtight...).

Perhaps something to do with the pressure valve (the fuel vapors thing), but there is no other fault code.

If anyone has a clue, I'm interested

Cheers

 

Those ohms values seem way too high for the heater. If not the heater I doubt ohms is much use as other than a "definitely bad" test. You could use OBD but the code tells you something's wrong and it's normally the sensor.

It's critical to get the right kind of course and ebay is not always the best place!

 

Aw. Dammit.
Not sure why the P1646 came right now though, since the sensor has been there for over a year. Fuses are ok.

Nevertheless, that only hints at the heater element not being too good, but once it's at the good temperature (heated by the exhaust gases), the sensor readings *might* be ok? Or not at all?

Am I better off disconnecting both sensors altogether until I purchase proper ones?

It failed the emissions test at last MOT, and I'm supposed to pass it again next Saturday. I doubt it will give good results.

 

When they don't work (even if they're not bad enough to flag codes) there's a serious risk to the cats - which are costly.

I'd work through the workshop manual or if that seemed too onerous put in known good sensor(s) but bear in mind the full list of things which could be wrong (which are usually itemised in the Jaguar doc).

 

I understand. I'll look for replacement sensors.
Just hoping the RH one didn't already trash the cat, as it's been in place for a while.

Thanks for the tips.

 

Quick update : I monitored the short term and long term trims on the way to work.
Initially, bank 1 had a STFT of 25%, bank 2 was 0%.

Once the engine was warm, both were 0%.

Long term trim on bank 1 is still 19%.

I'm guessing the heating element is bad but once the sensor gets heated up it works and so do the trims.

Or maybe not. The MOT will tell...

 

Hot engine, parked, at idle. All OBD monitors should be set (so, pending P1111) and LTFTs within about 5 of zero.

Driving is tougher to say much about unless you want to get into LOAD etc maps. (Don't let me stop you researching!)

 

So I went to the MOT, not good as expected - the fault code is enough to fail the test.

As per the emissions: the lambda value reaches 1.33 at 3000 rpm, but is ok at idle.
Above 1.04 means the air-fuel ratio is on the lean side, right?

I'm not sure of the conclusion to draw here. Just the sensors are bad? (I retested them and got 8 ohms on each this time, should be 2 as I understand. My meter had allow battery earlier).

 

You might have a bad ECU. From what you’re describing, that what it sounds like to me.

I know it’s a pain, but swap the sensors and see if you can move the issue to the other bank. If it moves with it, then I’m wrong. If it doesn’t, I’m probably right?

 

That thought actually crossed my mind, especially since when I plug the OBD reader, after a few minutes the connection fails and I can't get it back until the next restart.

I swapped the sensors indeed, didn't really change the error. In fact it was P1646 initially, then switched to P1647, now back to P1646.

Today I removed both sensors and ordered Densos (DOX-0430). If the issue persists I'll send them back and put the previous ones... I checked the workshop manual and it says the heater resistance should be between 4 and 10 ohms, mine are 8 so should be ok (workshop is for the 98 model, mine is 2000 but I doubt the parameters changed so much).

Is there a definitive way to diagnose a bad ECU ? Also that doesn't sound like it would be a cheap repair... A used ECU itself isn't expensive, but needs reprogramming as I understand, and I'm guessing I can't do that myself.

 

The fault moved with the sensor so it's virtually impossible it's not the sensor. Why even think it's not...

 

Well it changed once, then it didn't. Even disconnecting the left one, I wasn't getting the P1647 anymore, at least until I disconnected the battery.
Nevertheless, I'll see what the new sensors give. If I receive them at all that is, an emergency state was declared in France and I'm not sure the delivery services will keep working during the upcoming weeks...

 

JagV8 is correct. The fault moved, so it’s a bad sensor. Sometimes the new ones are defective.

 

Final update, or so I hope : I got the new sensors - Denso DOX-0430 -, mounted them.
Error code was still present, so I erased it and unplugged the battery for a day or so.

Fortunately, it didn't come back, at least so far - I started the engine 3 times since then.
Now to the MOT and hopefully it will pass this time.

For the fun part : the measured resistance on these Denso sensors is even higher than the previous ones... Getting 10.4 ohms. Yet it appears to work fine.

 

Don't go for the MOT until you get P1111.

 

I never got that one actually. Maybe my code reader does not report it, simply (an iCarSoft 930)

I'll see what the results are anyway, I can't postpone this too much.

 

Passed the MOT successfully. Lambda is 1.01, error code no longer present.

So again, the conclusion is that when it comes to replacing the O2 sensors, being a cheapskate is NOT a good idea

 

The pass is good news!