Where should I start?
 

Hi All,

I took my 2002 Discovery SII to an Advance Auto Parts and had my codes read out, these are:

P0300 - Random misfire;
P0303 - Cylinder #3 misfire;
P0306 - Cylinder #6 misfire;
P1300 - Catalyst damaging level of misfire on more than one cylinder;
P1562 - Battery supply below 9V while engine running; and
P1590 - ABS rough road signal circuit malfunction.

Where should I start tackling these issues?

Any help will be very much appreciated! ;-]#

The battery.

test your battery and charging system first. If that is not working up to snuff the rest of the issues will not matter, as your truck will not run.

Thank you for steering me towards the battery issue, but if "P1562 - Battery supply below 9V while engine running" is the reason why you recommended to check the battery first, please know that for a while I was running my Disco with the eBrake handle getting hot, the "M&S" lights blinking, the radio frequently going silent, and inside lighting flickering like if power wasn't good; all these pointing to some kind of electrical issue. While all this was going on my battery died and had to replace it.

I was recommended that it might be a grounding problem, and that I needed to make sure that the braided strap connecting the engine to the firewall was tightly in place. Well it wasn't, the nut was missing. So after fastening it, all related probs seem to have gone away, so I'm pretty sure maybe that was to blame for the P1562 error.

13.5+ volts

Ok so have you cleared the codes and gone for a drive.... any other codes still popping up?

x1. Get a load test on your battery at an auto electrical shop. Alternatively, a quicky test, put an AVO/Fluke meter set to 20 volts across the battery terminals and then get someone to start the truck. If the volts drop below 9V whilst the starter is engaged there is defo an issue with the battery. New battery is probably 100$ so get the biggest cranking amps poss consequent with the same physical dimensions.

...nope, nothing has been cleared, don't know how to; those are actual codes appearing, even after the hot eBrake/grounding wire issue being fixed, which I think was what brought up the P1562, thanks.

...checked out the battery myself with a multimeter, reads over 12.3 and with the car and everything running between 14.0 - 14.3, thanks!

you can't check a battery with a multimeter.
Well, you can check that the cells have 13.8 volts when running
and 12 volts with the truck turned off.

I would put in a new battery.

reason..
If the battery is partially shorting - it is taking a lot of current and boiling and has heat in it.

Time to junk it .

This will drag the system down in voltage and the ECU will throw codes.
All the systems are meant to run on 12V and above.
Especially the transmission for shifting.

Don't be penny wise and pound foolish.
Junk that junk battery.