The P1646 mystery

X300
1

After a recent hard driving session I got MIL and a P1646. No restricted performance, nothing noticeable changed. Also if you deleted the code it would not illuminate again until next startup.

All good and well, not the first time I get a trouble code.

Things got a little more hairy when I opened my X308 workshop manual to troubleshoot the code and found that the P1646 is a secondary fuel pump relay issue, pertinent only to the supercharged version. Mine is the non-supercharged 4.0 V8, I don’t believe I have a secondary pump. So, what’s happening here?

I’ve red reports of the same code being used as a pass side upstream O2 sensor, but no mention of that in my manual. Is the info reliable? Don’t want to be forking out for a new sensor, let alone going through the pain of removing old one etc. if the one I got is good.

Here’s proof:

Screenshot 2022-02-15 at 16.01.39
Screenshot 2022-02-15 at 16.01.392088×348 32.7 KB

2

what year is your car ?

3

It is two thousand and one

4

P1646_X308.pdf (162.7 KB)

Maybe try a normal BROWN relay in the socket?
If the fault does not recur, then call it GOOD?

1 Like
5

Sorry, I don’t follow. Try a brown relay in which socket?
Thx

6

The socket slot that the #2 fuel pump relay would normally go if the car was SuperCharged.

7

Thats an interesting idea. I’d try it but if it worked it would confuse me even more!

8

Can’t help you with strange electrical faults over the internet but electronics have no answers at times! Maybe the harnesses have a ‘queertron’ infection?

9

Haha. Nah, not a big believer in magical thinking me.

10

i suspect this is the upstream exhaust sensor (with grey plug)

HO2 Sensor Control Malfunction Bank 1 Upstream