ECM-4801 Diagnostic trouble code (DTC) information
Condition: The control module continuously checks three-way catalytic converter (TWC) efficiency by comparing signals from the rear heated oxygen sensor (HO2S) with the front heated oxygen sensor (HO2S). Diagnostic trouble code (DTC) ECM-4801 will be stored if the efficiency of the three-way catalytic converter (TWC) deviates too much.
Substitute value: Rear heated oxygen sensor (HO2S) control is disabled.
Possible source: misfiring, air leakage in the intake system, air leakage in the exhaust system, faulty fuel pressure, uneven compression, defective three-way catalytic converter (TWC).
Fault symptom:malfunction indicator lamp (MIL) lit.

Was VIDA (or VADIS) used to get the code or was something else connected? See you mentioned another tool being used to reset the code so just asking. With VIDA, you can get a number of values for the sensors as well as doing some active troubleshooting. I'm frankly surprised, if VIDA is being used, that this is your only code.

 

I had (have) the same problem when codes p0420 and p0430 showed up. **ODB P0420 = ECM-4801 *

I took my XC to an indy shop who diagnosed the issue as a failed front O2 sensor and I had him replace it. The codeS were cleared and didn't come back until a number of miles later when the p0420 code re-appeard. I then replaced, myself, the rear O2 sensor (both bosch) and the code went away then came back some miles later.

http://www.obd-codes.com/p0420 indicates possible causes :

- Leaded fuel was used where unleaded was called for
- A damaged or failed oxygen sensor (HO2S)
- Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
- Retarded spark timing
- The oxygen sensors in front and behind the converter are reporting too similar of readings
- Leaking fuel injector or high fuel pressure
- Cylinder misfire
- Oil contamination

Possible solutions listed :

1. Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
2. Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
3. Inspect the downstream heated oxygen sensor (HO2S), replace if necessary
4. Replace the catalytic converter

The sentence that caught my eye on that page "Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily."

Like a newb, I went straight for replacing the O2 sensors. I've left it alone for awhile because I didn't see any difference in how the car drives. HOWEVER, from what I've read, it's possible the Cat could be damaged which I hope is not the case since that is a pricey fix. I'll do the checks recommended and hope it's an inexpensive leak. I've had the spark plugs replaced at a stealership prior to this happening. Now that I recall, they did replace one of the solenoids so maybe it's related to that. I've had silicone hoses put in when I put in the IPD TCV but that was back in 2008. I also put in the silicon coupler hoses around that time.

PS. Another post in the R forum indicates :

Possible source

-misfiring
-air leakage in the intake system
-air leakage in the exhaust system
-faulty fuel pressure
-uneven compression
-defective three-way catalytic converter (TWC).

Another poster in that same thread has this opinion :

"The rear O2 sensor is a crude instrument, capable only of sensing whether the mixture is "going lean" or "going rich", i.e. it can detect the crossing but not the absolute value.

The front O2 sensor on the other hand can measure the mixture precisely, and that's exactly what it does for the ECU. The ECU uses it for fine control, and the rear one for trending and catalytic converter operation. But, it does this by comparing the front to the back, with values as would be expected from stock hardware. If you change the hardware, you change the response, and if that response wanders out of margin, you get a CEL.

The CEL in itself isn't the problem. The problem is that a) the car will fail inspection since it's signalling a fault and b) the car stops trusting its sensors, meaning it goes into a single-sensor mode (which can't adapt), open-loop mode (which is inefficient and smoggy), or limp mode (which nobody wants). " ... in the aftermarket cat context.

My XC passed California Smog ... I tried clearing the code before getting the inspection but the shop busted me and told me I had to drive until their instruments indicated it was ok to tak a reading which was about another hundred miles. Luckily for me I was able to get the inspection after their instruments said it was ok but before the CEL came back on. The point is, my emmissions were OK with the sensor reading being our of tolerance.

 

Hey guys thanks for the info. Good stuff. I do not have the VIDIS system although every time I need to do a repair I think I should make the investment. I talked on the phone with my repair guy today and his opinion over the phone was; if the car seems to be running fine than its probably an air leak or the rear O2 sensors. Apparently the converter is rarely the culprit when this code is thrown. I'm bringing it in for a diagnostic test in about a week and a half and when I know more Ill post it here. Without the use of a smoke machine is there a way to test for air leaks in my driveway? Also thought I'd throw in that I've averaged 16.6 mpg on premium since the code came up, so the last 2 to 3 months.

 

Check this out : http://www.ipdusa.com/products/10180/121524-turbo-intercooler-hose

In the product description, it says :

"Volvo utilized a plastic pipe for the "over the engine (OTE)" pipe on these models. Most of the time this is fine. However, over the last few years we've seen more and more of these develop cracks. Particularly near the hold down tabs. Turns out, this guy makes for a great handrest for the oil change guy when he's leaning over your engine bay. This leads to stress cracks which can then lead to deceptively hard to diagnose check engine lights and performance issues due to the vacuum leak created by said crack."