Engine codes P0420 P0430 | Ford Explorer Forums - Serious Explorations

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Engine codes P0420 P0430


New Member
January 11, 2011
Reaction score
City, State
Salem, Missouri
Year, Model & Trim Level
1998 Explorer Sport 4.0L
I have a 1998 exploder sport showing codes P0420 and P0430.The code book says, catalyst efficiency below threshold (bank1) and (bank2) Does anyone know what that means?:exp::usa:

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It most likely means you have a bad exhaust leak at both manifolds or your cats are bad.

The exhaust is quiet with no leaks.

Bad cats then...

Don't jump to replace cats just yet

Often it is a simple tune up or the rear O2 sensor that will throw "one" of these codes (bank1 or 2) but to have both left and right codes at the same time, leads me to think of the cats as an absolute last resort. They usually won't fail at the same time, in fact after all my years at Ford I don't think I've seen that once. Check and clean connections to REAR O2 sensors, especially if you off road it (they will generally throw their own code but could be operating just within threshold and heating up okay, but due to poor connections not reading the post-cat output quite right.

Excessive oil burn, blow by or ring wear CAN lead to it too but there are many other simple and less expensive things to try first.

As jremington said, could be an exhaust leak, I've seen many that are barely noticable but present all the same.

Replace plugs and wires, try running Seafoam through it. I don't normally fall for miracle in a bottle solutions but Seafoam really does work well for cleaning out intake carbon, I Seamfoam annually just as regular maintenance. Replace PCV valve etc. I've found basic tune ups usually clear out the cat codes.


+1 for ozmedia's strategy!

Based on a dealer diagnosis, I replaced the cats (all 4), 92,500 miles, '98 AWD to the tune of 1300 at my shop. At about 120,000 miles, was getting codes again and was told it was the cats again, and they were just out of warranty. Got smart, got a code reader. Showed P0420 and P0430 codes. Eventually got a P0153 code, which is Bank 2 O2 sensor 1. I replaced all 4 O2 sensors at 140,000 miles and have driven nearly 40,000miles without a code until just recently started getting P0430 again. I plan on spending about 60 bucks for Bank #2 sensors before replacing any cats!
My suggestion is to replace the O2 sensors. There is a wonderful post on how to replace 3 of them by removing the center console. 30 to 40 bucks apiece sure beats 1300 to get a car to pass inspection.

O2 sensors are more likely to go bad than the cats. And are cheaper to replace (for a quick test).
Usually the upstream O2 sensors are all that you need to replace. That did clear my P0430 in my Taurus...

Replaced upstream O2 sensor only

Did exactly that about 1000mi ago, code is GONE! Thanks!