Code 0521 is the read out. What does it mean? I cannot find it in the catalog.

2POSTS SAME TOPIC WHY, o2.
MY GUESS IS YOU ARE ASKING HOW TO BUY THE 2 SENSORS
WELL ALL SENSOR MAKERS ON EARTH TELL THIS PER CAR.
EVERYONE
I BUY ONLY BOSCH SENSOR (THEIR INVENTION ,FACT)
AND THEIR E-CATALOG IS FREE T READ 24/7/365
HERE IT IS FOR YOUR BOSCH DS RIGHT. B1S2 15719
MOST CARS FRONT AND REAR ARE NOT THE SAME
THAT MEANS UPSTREAM AND DOWN NOT THE SAME BUT GEE i NEVER EVER GUESS I READ HE CATALOG
V6 GAS ONLY AND NOT HYBRID '

SOME MAKERS OF 02 HAVE DIFFERENT RULES!!!
SO i WILL ONLY SAY FORDS RULES PARTS THEIRS ONLY
L AND RIGHT REAR. ~444AC SEEMS ONLY REARS ARE SAME.

https://www.oemford.parts/oem-parts/ford-oxygen-sensor-yl8z9g444ac?c=Zz1lbWlzc2lvbi1zeXN0ZW0mcz1lbWlzc2lvbi1jb21wb25lbnRzJmw9MTAmbj1Bc3NlbWJsaWVzIFBhZ2UmYT1mb3JkJm89ZXNjYXBlJnk9MjAwNiZ0PWxpbWl0ZWQmZT0zLTBsLXY2LWdhcw%3D%3D

REAR = S2 AND REAR OF CAT. = DOWN STREAM.

IN FORD PARLANCE (TRICKY) LEFT = BANK2 ,RIGHT = BANK1.
THEY MEAN RIGHT CAT IS BANK 1.

I couldn't find engine code for 420 but may be body code take to AutoZone for free test and code meaning

What model of Ford? With that I can tell you.

P04ef is not a vaild code. codes are one letter and four numbers.

I've got a list of some OBD codes, hope it helps someone.

Mercedes Benz OBD codes

did you try , HONDA?
if not at rockauto.com , that means , usually its not made aftermarket. (fastest way on earth)
SMP is a large maker of relays for cars, and they do have a e-catalog. read it.

-You mean what is ( that code) mean?
-If so look in its repair manual.
-Or if you mean what and how to read the code from the car?
-If so. Need a scanner.

mai Lor

When i see that on my scanner it means to hit the `enter' button and it will read the codes out for you.don-ohio