I pulled both of these codes the other day from my 1995 xj6 4.0 and need a little guidance on what to look for. The car has not been driven much over the past 2 years. However, it runs great!

Gus:
That's a general o2 control fault. You imply it came up on both banks at nearly the same time, so I would bet it is not sensor or cat related. That leaves a MAF problem (have you cleaned it lately?) or an air leak in the induction plenum. The manual says exhaust leaks are another source, and cracked exhaust manifolds causing O2 havoc is also a known problem on these cars.

Thank You! Like you I am thinking exhaust leak and have not looked into it yet. I did not however, think of the MAFS. I will look at both and see what I can find. Where are the common leaks on the exhaust? What do I look for?

Gus:
The manifold cracks (under the shiny trim piece). There are not many scrapper ones intact, so welding is the best bet. Don't bother with hi temp epoxy.

Thank You! I will post my results.

These codes are catalyst effeciency codes, not general O2 sensor codes.
P0420-catalyst effeciency below threshold bank 1
P0430-catalyst effeciency below threshold bank 2
Bank 1 is Cyls. 1-3
Bank 2 is Cyls. 4-6

Although exhaust manifold cracks are common on these cars, these codes are most certainly caused by your catalytic converter which is bolted to the exhaust manifolds. If you hook up a datastream tool of some sort and monitor the downstream 02 sensors when the engine is nice and toasty, you will notice they are probably swinging from about 0v to 1v. This should NOT be happening. They should read approx. .5 volts when idling and should not fluctuate. If the 02 sensors are fluctuating, the catalyst material is compromised and not doing it's job......cleaning up exhaust gasses. I take it this vehicle has high miles, say above 60 or 70k? If so, replace the cat. We do them frequently. If you don't replace the cat, and decide to clear the codes, they will most certainly return with a quickness.

poboyblues"
Unless ther ehas been a contamination event, it is highly unlikely that two cats will go out on these cars at the same time. And my cats, as well as many other's have gone for more than 200,000 miles. The code means the front lambdas do not match the back, which can come from bad cats, or from bad sensiors, or from a leak, or from being outside the operating range. And that is all in the shop manual, and makes good sense. I stand by my advice on this.

"If you hook up a datastream tool of some sort and monitor the downstream 02 sensors when the engine is nice and toasty, you will notice they are probably swinging from about 0v to 1v. This should NOT be happening. They should read approx. .5 volts when idling and should not fluctuate"

This is absolutely wrong!! The transition time, not the absolute value is the controlling constant. If the sensors are not cycling (either upstream or downstream) then something is wrong.

OK sparky, take it away.

I have enjoyed your comments and you have given me something to do once I get my 99 xk8 opportunities completed. One thing these cars have given me is an opportunity to work on them.
Thanks Again!

I have an interesting situation. My check engine light comes on after getting gas, so I changed the gas cap and it is still on. It goes off after the tank goes below 3/4 or 1/2 with no consistency. Recently my car had a gunked up Mass Air Flow Sensor causing the car to stop in the AZ Desert. The code is P0420, but one cat has been replaced already. Any clues?

So a P 0420 code bank 1 below specification - would it be the front O2 sensor that would be a good bet to replace ? I see one O2 sensor on each section of the exhaust manifold

these front o2 sensors seam to be the most common part available so I'm guessing it a common issue... Bank one I'm assuming would be the sensor closes to front of the car ( corresponding to the first three cylinders )

I forgot - my car is 1995 xjs 4.0 -

The code 420 is Cat efficiency which is the rear O2 sensor . There is a mod to twek the reading to eliminate the code . Bocsh has a connector ready equivalent that is much cheaper the Denso . Look for corrosion at the connectors .