Hey all, I'm new to this so I hope I get it right. Just purchased an immaculate 2000 XKR with less than 40k miles. The other day the MIL light came on. Read with an OBD scanner and came up with a P1646; Linear O2 Sensor Chip Bank 1. I dug around on the Internet discovering this forum and found a link to Jaguar documents, one of which was entitled "On-Board Diagnostics, AJ-27 Engine Management" for the 2000-2002 SC engine. I found another that was exclusive to 2001 engines. Both of the Jaguar documents specifies P1646 as "Fuel Pump Relay 2 Malufunction". Hence, I am confused.

Does anyone know for certain which is truly correct? Does anyone know why the Jaguar documentation might be incorrect?

 

Welcome!

I see you have found your way around the forum already. Sounds like you have a nice car...we love pictures!

Stick with the Jaguar-specific code definitions. They can vary a bit year to year so continue to be specific.

Invest in a JTIS manual from eBay and hang around here and you will become very knowledgeable fast.

 

Welcome to the forum!

At the top of this page is a FAQ Sticky that has a link near the top to a free download of JTIS. Imbedded in JTIS is the Jaguar specific OBDII codes for each year.

 

There are two types of DTC codes, 3 digit and 4 digit.

The three digit codes ( those that look like P0XXX ) are required by law to be defined the same for all manufacturers. This was part of the 1996 OBD II standard, in order to aid non-dealership mechanics in diagnosing problems. They ALWAYS mean the same thing no matter what car the code comes from.

4 digit codes (PXXXX) are manufacturer specific, to allow the manufacturers the flexibility to define codes for specific problems that might be unique to their vehicles. So your P1646 means 1 thing on your XKR, and very likely would mean something completely different on a car from a different manufacturer.

In short, go with the Jaguar documentation.

 

Thanks for the insight guys...appreciate it. Given this car has two fuel pumps I am curious what the symptoms are when the secondary pump fails. The car drives well when I get on it, and I can't sense a lack of preformance, only a slight roughness when idling and a small amount of vibration otherwise. Thought I might spell out the steps taken thus far as I may have missed something. Any insight is appreciated.

1. Got MIL light after starting (car was not started for two days prior). Did not get "Restricted Performance" message.
2. Got P1646 on scanner..no other faults. Care drives well even when I get on it. Seems the car is idling a bit rough.
3. Pulled relay. Got the restricted performance message.
4. Replaced relay, same P1646 on scanner. That's wher I am at now.

So does anyone have an idea what to expect with a secondary fuel pump failure that I can detect? Did I miss something? Pulling the tank seemd like a pretty big mistake if I am wrong.

 

There are two relays for the two pumps. The relay for pump 1 is in the actual trunk fuse box, and the relay for pump 2 is by itself hanging next to the fuse box, it also has another fuse, have you checked both relays and both fuses?

Before you get into pulling the tank, you might check the resistance from the relay wiring to ground through the pumps to see if one of them is high resistance.

I'm not sure how the engine controller detects a fuel pump failure. I got this Code once on my car, but only once out of several times reading the codes, so I am not sure I believe it.

 

I am not sure that I agree. I see the P1646 as an H02S sensing circuit. Please correct me if I am wrong.

 

The electrical diagrams I have only show a fuse for the primary pump. The diagrams I ahve are pdf for the 2000 XK8/XKR. I don't have JTIS so if there is an update I am unaware of it. Where is the fuse?

Regarding the possibility of it being an O2 sensor. That is what the scanner indicated, but the Jag DTC code documentation indicates it is the Fusl pump (see 1st message in thios thread)

 

Maybe this will help.

Basically,

4.0L cars, SC/Non SC, P1646 = Fuel Pump 2 Fault
[of course, Non SC don't have a fuel pump 2]

4.2L and V6 Cars, P1646 = Heated O2 Sensor Fault

Per attached document images.

Cheers,

 

Gus, I am going by a document called "Jaguar Onboard Diagnostics AJ27 Engine management." AJ27_EMS_OBDII.pdf

This covers the 4.0L SC engine in the 2000 - 2002 XK. ON page 58 it defines P1646 as "Fuel pump relay 2 malfunction." Since the 4 digit codes are defined at will by the manufactuer, they sometimes mean different things on different models, or different model years.



The two fuses are in the trunk fuse box.

Pump #2 relay is fuse#3 5Amp. If the fuse is OK, you might testing by swapping the relay with another one. The primary pump is Fuse #7 a 20Amp.

 

I don't have the correct permissions to see the JPG's. I do believ tough that what you were trying to show me is the same engine management info that I have...thank you.

I ended up finding the fuse...good call, but it was fine. Looks like the next step is a pro...pulling the tank is too much for me... amd the car is too pretty for me to screw up trying. Thanks all for the support!

 

A Fuel Pump Relay Fault is triggered when the ECM sees a open circuit in the Fuel Pump Relay COIL. I would replace the Relay or test for a open wire from the ECM to the Relay.

The ECM checks for Current Flow in the Relay Coil. It does NOT monitor Current Flow in the Pump Itself.

 

Thanks for the insight. I think I did perform a check of the circuit when I pulled the relay. Follow what I did if my grammar isn’t pathetic. I had only the MIL light and P1646 when this started. I replaced the relay with a new relay and no change, same MIL light and same P1646. I referenced a 2001 Jaguar OBD code document and discovered that in 2001 a failed relay coil circuit would cause the same P1646 plus a message to appear in the odometer screen that says “Restricted Performance”. The explanation of ”restricted performance” is RPM is limited to under 3K.
I asked myself what would change from 2000 to 2001 because I had the P1646 but didn’t have the restricted performance message. I decided to clear the P1646 message and then pulled the relay to fully open the coil circuit just to see what happens. I got the same P1646 message, except this time had the restricted performance indicator as well. Why the change??? If the ECM only monitors for an open circuit, why the change?
I did not yet check for 12V volts at the relay coil, nor did I monitor if the ECM is outputting a ground to enable the relay, but I will. Fuses are all good and brand new relay in place. Electrically an open is an open, so why the difference in indications between the relay installed versus the relay removed? My guess is more is being monitored here that the OBD documents indicate. Maybe the ECM id bad???
Again, thanks for the help. I am finding that the discussions on this forum only help to educate….

 

I am still confused. I have a 1999 4.0 and the chart reads P1646 as an 02 sensor. I do not have a 2000 chart so I will heed to you!
http://www.gusglikas.com/images/Auto...J27%201999.pdf

 

Did you ever clear the codes, aside from when you tested it with the relay removed, to see if the code comes back with a new relay in place?

As I said I got that code once on my car, but it hasn't come back. Maybe it's just a phantom.

 

Y'know, Gus, we have always been directed to rely on the detailed OBDII diagnostic documents as the final word on codes and parameters etc. that set them. Plus we have always been supplied revisions to code charts, such as the one you are referring to. In other words, the chart I go to if I'm researching a 1999 XK8 like yours, says fuel pump 2 fault instead of HO2S fault. But the more I dig and find the code mentioned in bulletins, diagnostic aids etc., the more conflicts I find. I have now found 4 DTC summaries that are supposed to apply to the AJ27 system, and P1646 goes back and forth from one chart to the next. This is nuts.
Next week I'll be with some technical people that I can put the questions to. I'll see what they can come up with and I'll report then.

The only thing I will say is, to date any P1646 I have faced, was attacked using the documents I posted the images from; and I haven't been led astray as yet. Meaning if the document said Fuel Pump 2, that's where the repair was made to correct the fault, and vice-versa. Granted it has only popped up maybe 5 or 6 times, I'd guess.

So, thanks for being persistent; we'll see where this leads. I HATE conflicting technical information, makes us look like idiots sometimes.

Cheers,

 

Steve,

Do not misunderstand my position, all I want to do is give the most accurate information possible. My disagreeing was not a reflection on you it is a call to check. I, like you, do not like giving out bad information.

 

No, no, I didn't misunderstand.........When I said thanks for being persistent, I meant it. This has come up before, and I had satisfied myself it was hashed out, 16-bit systems one way, 32-bit systems the other. You caused me to dig deeper and I find more conflicting materials that I call unacceptable. I figure I'm the one with access, so I need to find out what they have to say about it. I'm basically their representative, so to speak, and I don't like getting caught with my pants down!

Soooooooo! Thanks again!

Cheers,

 

Interesting thread guys I have been having a similar set of problems on my 2000 my XKR, first off I have hesitation during kick down acceleration and when it tries to drop down a gear, from a standing start it appears to go through all of the gears ok (mostly still has a slight hesitation now and then).Did check on scanner but no codes.
Today had a P1646 code on both my scanners which indicated the "linear 02 sensor control chip bank 1" so good job i found this thread might save a bit of cash if it is only a relay instead of a 02 sensor, I cleared the code and as yet it has not flagged up but will check the relay tomorrow, so thanks for the info

 

glad you found the older thread helpful, there are some real gold mines buried in the forum's historical threads.