as it relates to P0420/P0430 Catalyst Inefficiency Codes
When a vehicle is exhibiting one of these codes (or variations of these, such as P0421/P0431) follow these simple steps.
First, if ANY other OBDII codes are present, address those first. (Evap. codes, Misfire codes, O2 sensor codes Lean/Rich codes, Timing Control codes, etc). These can all cause false catalyst codes to be set when there is actually nothing wrong with the converter itself, so if any codes other than the P0420/P0430 family of codes are present, start with them!
Before blaming the catalytic converter, regardless of the age of the converter, number of miles on the converter, or the brand, a few simple operations with your scanner will help to eliminate costly comebacks.
Hook your scanner up in OBD II GLOBAL MODE rather than the vehicle specific mode using the VIN. This will ensure we are working with the same information scales. Be certain the vehicle is running at operating temperature, and the computer is in closed loop.
Collecting and Understanding Scanner Data
In live data, find the Long Term (LTFT 1 and LTFT 2, etc/) and Short Term (STFT 1 and STFT 2, etc.) Fuel Trim information.
In OBD II GLOBAL, the fuel trim data will be presented as a positive or negative number with ZERO being the center point (1). A fuel trim of 0% is perfect, but unrealistic, some amount of trim will always be acceptable, and even required by the ECU (electronic control unit) or PCM ( power train control module).
The OBD II parameters for fuel trimming are generally 25% either positive or negative before setting P0171, P0172, P0174 or P0175 codes, but catalyst codes will be set long before the trim gets to those numbers.
When adding the long term trim to the short term trim, you obtain a basic “total trim number”. This number should be very close to zero, never exceeding 10% positive or negative.
A LTFT of 3% and a STFT of 1% would give a total trim of 4% which is within the normal range.
Negative numbers can work in your favor as well; for instance, the STFT is 4% and the LTFT is -2%, the total trim number would be 2% which is near perfect. The same thing if the total trim number wound up in the negative.
A number close to zero is fine, but 10% negative or positive is out of limits.
Now, besides the total trim value being well below the 10% threshold, the spread between the long term and short term fuel trim values must be as well.
As noted above, adding a negative number to a positive number can get you closer to zero which is a good thing, but if the spread is too large, you need to find out why.
For example, LTFT is 7% and STFT is -7%, well that would give a total trim number of zero which is “perfect”. However, the spread between the numbers is 14% which exceeds the target “10%”. While it is true the total trim number is zero, it’s a bad way of getting there. It is an indication that the PCM (power train control module) is not in complete control of the fuel injection, it’s scrambling to make up for an issue that is nearly out of its control. If the vehicle utilizes mass air flow (MAF), this condition STRONGLY suggests Mass Air Sensor issues. If the vehicle utilizes MAP (manifold absolute pressure sensor), check the vacuum at the MAP sensor.
Suppose that the numbers are all well within the target 10% limits at idle (remember, that doesn’t mean 9% is good, just that the numbers may not have exceeded 10% yet). You should now make certain that there is no fuel starvation issues occurring.
Hold the engine RPMs up around 2500 for a minute and observe the trim data once more. (Hint:) if the numbers don’t change, that would be very suspect. Short term trim will usually be moving all the time, but if your Long term trim doesn’t change from idle to 2500, you need to investigate further. (This does not hold true for VW and Audi, their Long Term Fuel Trims will not change unless the vehicle is moving at a cruising speed).
Now check your results.
If the numbers are well within 10%, replace the converter along with the upstream oxygen sensor — you should not encounter any further problems. Remember, oxygen sensors have a useful life of approximately 80,000 miles. Even if you went to the trouble of connecting a digital scope to the signal wire and observed the sensor switching across 450mv within the 200 millisecond parameter during a drive, what happens when the sensor exhibits a slowness to switch due to age and deterioration, allowing the vehicle to run rich for too long a period? It could destroy the new converter. So, ALWAYS replace the upstream oxygen sensor when replacing a converter unless you personally know the sensor has much fewer miles on it.
If your results are NOT within the 10%:
A positive fuel trim denotes a lean condition. A lean condition can mean too much air (un-metered air), or an insufficient amount of fuel.
A negative fuel trim denotes a rich condition. A rich condition can mean too much fuel, or an insufficient amount of air.
If your fuel trim # keeps increasing (leaner) as your rpm increases, you should suspect fuel delivery issues or a massive amount of unmetered air. (ex. bad Evap Cannister).
If your fuel trim is lean at low RPM, but decreases with an rpm increase, a small vacuum leak could be suspect.
One final note. After replacing a catalytic converter, and resetting existing codes, there are sometimes issues getting the readiness monitors to set. If the problem persists for hundreds of miles, you need to realize that most vehicle’s monitors set in the order listed, so if any monitor is listed before the catalyst in your scan data, the catalyst monitor will not usually become ready until the prior system’s monitor does.
If the prior system never becomes ready, neither will the catalyst, so again, address any un-set monitors prior to the catalyst monitor before spending time trying to get the catalyst monitor to become ready. Unfortunately, we’ve seen many hours wasted trying to complete drive cycles only to find that there is a computer problem at the root of the monitor issue.
On certain older Nissan products, the center point is 100.
On certain older GM products, the center point is 128.
The center points now become the zero point and the above rules then apply.