Po420 & po430

[stoneone25]

I have a 98 ford explorer with 200,000 miles on it. It has the
4.0 sohc engine

When i start it in the morning since it has been getting cold
it rev to 2500 rpm when first started then will drop to around
500 then it goes up and down between 500 and 1000 rpm.it
sounds like it is not getting fire in all cylinder. runs rough.
sounds ok when reved up. After it warms up run good except
for a small miss when it cold outside.

Getting code
PO420
PO430
Po174
Po171

I have replaced spark plugs and wire
Maf sensor
Iac sensor
fuel filter

Don't know what to try next.
should I replace the O2 sensor. Don't know if that is the
problem since I'm getting code PO420 and PO430

Could it be the TPS?

 

[2000StreetRod]

Lower intake manifold

P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0430 Catalyst System Efficiency Below Threshold (Bank 2)

Your lower intake manifold gaskets are probably leaking. As the SOHC ages the gaskets dry out and leak when the temperature is cold. The two gasket sets cost less than $30 and replacing them is not very difficult but is time consuming. See the thread Starting my 00M12 Installation and ignore the parts about the timing chain tensioner and restrictor.

After you've fixed your manifold leaks the other DTCs (420 & 430) may no longer be present. Does the vehicle have the original catalytic converters? At 200,000 miles the cats may be getting depleted.

 

[stoneone25]

I replaced the upper intake gaskets. Didn't replace the lower yet. It has the original
cataltic converters. But since i'm getting both Po420 & Po430 i'm thinking it something else causing the errors. It started reving to 2500 rpm after I replaced the IAC valve.
Had error po420 before I replace the IAC. Now it shows both po420 & po430. Might put the old one back on and see what errors shows up. Could a bad tps cause the rpm to go up and down between 500 and 1000 rpm when cold. Idle ok when warmed up just a little miss.

 

[2000StreetRod]

wrong gaskets

The gaskets that typically leak the most are the ones between the heads and the lower intake manifold. Your cold start "flare" matches the symptoms of the lower gaskets leaking. I wouldn't worry about the cats until the lean condition is fixed. The TPS is easy to check: TPS Test Procedure
It may help to clean the IAC valve but it probably won't solve your problem. Make sure you put the IAC valve gasket on properly. Check all of your vacuum hoses for leaks.

 

[Wrench]

Don't reuse the old IAC O-Ring gasket!

 

[bikrboyr6]

I have a 97 4-door explorer with the 4.0 SOHC engine (nearly 200,000 miles on the body). The motor has roughly 100,000. I too am continuously receiving the PO420 Code (checked via the OBD scan tool). I reset it, and a day or two later it comes back. I have replaced the upper and lower intake gaskets which solved my rough idle issue. I do know that my muffler is leaking and needs to be replaced. I installed new exhaust gaskets when i performed a motor swap in nov. I would hate to have to spend the money on a new CAT (or 4 new CATS for that matter). How can I test the O2 sensors to eliminate them from the equation?