Chasing down cat codes

Jan. 1, 2020
The No. 1 codes in the country are the P0420 and P0430 (Catalyst Efficiency Below Threshhold) diagnostic trouble codes.

The No. 1 codes in the country are the P0420 and P0430 (Catalyst Efficiency Below Threshhold) diagnostic trouble codes. They cover the same problem, with the only difference being to which side of the engine they are referring. But there is more to be considered than the catalytic converters alone. Other key components involved are the O2 sensor heaters, the O2 sensors themselves, and the Powertrain Control Module (PCM) software that performs the catalytic efficiency testing.

What is catalyst efficiency? The catalytic converter is the last step in cleaning up any dirty gasses left over after combustion. Simply stated, its efficiency is a measure of just how well it can do its job.

A borescope can be used to inspect the condition of the substrate. Damaged substrate (broken, clogged, melted) means something killed the cat before its time.

Most vehicles (those certified to OBDII) have both a pre-catalytic (upstream) oxygen sensor (or air fuel sensor) and another oxygen sensor located after at least one of the catalytic converters (downstream). Conventional oxygen sensors respond to the amount of oxygen in the exhaust stream and provide feedback to the PCM for fuel control. But the downstream sensor should sense a relatively stable voltage signal. When this rear sensor voltage zigzags up and down at a steady RPM, it is an indication of a decrease in catalytic efficiency. The PCM programming determines how much zigzagging is acceptable. If the PCM interprets the readings to indicate a catalytic converter failure, a P0420 – P0430 pending DTC (Diagnostic Trouble Code) is first set and if the decreased efficiency is still occurring on the next consecutive test, a hard DTC is set and freeze frame data PIDs (Parameter Identifiers) are recorded and stored.

Remember, Not Just The Cat
As with any other diagnostic challenge, you must go after the root cause of these codes using a systematic approach. Even if you do determine that the converter has failed, there is often an underlying reason for that failure. Miss that, and the new converter will quickly suffer the same fate as the old one.

An often overlooked, but extremely valuable, source of information is the code’s Freeze Frame recording. Even if you own the OE scan tool, you should first make use of the Generic/Global mode of the tool. The reason? Generic/Global does not substitute engine values (PIDS). It also provides access to freeze frame data and Mode $06 test results if needed. The freeze frame data is going to be especially helpful to you in diagnosing these DTCs. Be sure you do not erase/clear DTCs since that will also erase/clear freeze frame. Freeze frame data might help you uncover a sensor value that is out of normal range only under the conditions that were present when the DTC was set.

Remember, though, that freeze frame data does not record at the exact moment the fault occurred. It can be in a 15-degree-frame window, meaning what you see in the freeze frame may have happened 15 degrees-frame before or 15 degrees-frame after the store screen. It can still be very helpful in duplicating the problem by providing some idea of the conditions the vehicle was under when the problem was discovered. Diagnosing and repairing a vehicle without freeze frame would be difficult, to say the least.

The next step in diagnosing any problem is in understanding how and why the PCM detected the fault. In most cases, the PCM is comparing the input signal from the upstream oxygen sensor to the input of the downstream sensor. As discussed earlier, the downstream sensor should remain relatively stable. And while an aged cat can cause the rear sensor to swing in synch with the upstream one, it is not the only reason why. And, as mentioned earlier but absolutely worth repeating, if the cat is indeed failed it is imperative to know why to avoid sealing the fate of the replacement.

After you’ve read and understood the conditions and criterion of the DTC(s) and before you even touch your toolbox, check your service information for any related Technical Service Bulletins (TSBs). In some cases, a software glitch has a reprogramming procedure available or a part has been upgraded. In either case, you’ll never know unless you look for the info.

Begin your work on the car by performing a good visual inspection. Are there signs the car has been maintained or neglected? Are there any obvious issues that could affect the PCM’s testing of the cat?

These are the top ten causes of catalytic converter efficiency codes and cat failures. You may need to go down the entire list before condemning the cat alone.

1.      Engine Mechanical Problem
2.      Cylinder Misfire
3.      Exhaust Leaks or Damage
4.      HO2S / WR (Oxygen Sensors)
5.      Fuel Pressure
6.      Injector Problems
7.      Oil Contamination
8.      ECT (Engine Coolant Temperature) Problems
9.      Intake Leaks
10.   Ignition Timing

General Testing
Every input and output on the car depends on a stable power source and dependable grounds. The first step in your diagnostic strategy, then, is to make sure the battery itself is healthy.

A hose held to the ear is one way to seek exhaust leaks that can cause sensor readings to be off.

I suggest the first place you start your code-specific diagnosis after the battery’s condition is verified is to check the overall engine health. This is accomplished by performing a relative compression test. This is easily performed by using a scan tool (if this option is available) or by using a labscope and high amp clamp. The scan tools perform this test in the 20-second range while the labscope test is completed in two minutes or so. You can see this performed by reviewing a few webcasts that are available on the Motor Age or the TST YouTube channel.

Another important piece of information to review very carefully is, both Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Fuel trim affects how much fuel the PCM commands the injector to fire. If the upstream oxygen sensor reports that the engine has a lean condition, the PCM increases pulse width to add additional fuel. To the contrary, if the sensor reports a rich condition, the PCM shortens the pulse width to reduce the amount of fuel injected. The STFT and LTFT numbers represent the amount of pulse width change, typically as a percentage. Zero is perfect fuel trim. Fuel trim that is positive means that the PCM command is trying to add fuel, and fuel trim that is negative means that the PCM command is trying to remove fuel.

STFT is an immediate response to the air/fuel mix being reported by the oxygen sensors and will move ideally across zero no more than +/- 10 percent. Personally I prefer to use +6/-6 when I diagnosis a problem vehicle. More shift than that indicates there is a problem somewhere. LTFT is an adaptive response to excesses in the STFT. It doesn’t swing across zero on your scan tool data stream like STFT does. Even so, excesses of over +/- 10 percent indicates a problem and depending on make, a more than +/-15 percent variance will set a System Lean/Rich DTC on most vehicles.

Air leaking upstream of or at the upstream sensor can cause the sensor to read a false lean condition.

Understanding fuel trims, and what they are trying to tell you is a learned skill that will save you time and effort on many driveability problems. For example, a Ford diagnostic flow chart for this code says that after confirming there are no other DTCs (an important step, to be sure), you should look at the O2 sensor wiring and then check fuel pressure. However, if we use a little common sense, if we are getting good O2 sensor and fuel trim readings on our scan tool, why would we perform these tests?

Don’t work hard, work smart. Remember that the most important tools are what God gave us; the brain, eyes, ears, nose and hands. So take a good visual look at the vehicle, look up TSBs and check out PID data carefully before ever following a trouble tree.

Some Case Study Examples
This one was a quick fix. Using the search function on our service information system, we looked up the specifics on the P0420 DTC on a 2001 Nissan Frontier. The search also provided us a TSB that informed us that when the catalytic converter was verified as OK, the repair was something other than replacing parts. This is not uncommon. I have seen plenty of vehicles where a new converter was blindly installed but the P0420 problem was still present or the onboard monitor responsible for keeping an eye on the cat would not complete its testing. These techs could have avoided a lot of frustration had they bothered to take the time to look up the TSB advising that a reflash was required for false codes.

This cat has obviously been abused, as witnessed by the molten hunks of substrate inside. Unless the killer is found and repaired, the new cat will soon suffer the same fate.

The next case study brings together all the basic knowledge of conventional oxygen sensors and Air-Fuel Sensors (AFS) that any tech should already know. This 2004 Nissan Altima 2.5L had a P0420 after some unrelated work had been done to it. The car was kept up with well and had only about 90,000 miles on the odometer. At this relatively low mileage, I suspected something more than just a faulty cat.

I looked up TSBs after performing the basic checks and found that the PCM may need to be reprogrammed. However, after checking the calibration codes of the car’s PCM, I determined that the PCM had the most up-to-date software all ready installed.

I used an exhaust gas analyzer to check the emission readings at the tailpipe. Interesting! They looked clean and correct. But that was in direct opposition to what the onboard test results were. I began to suspect a bad sensor, exhaust leak, worn converter or a bad PCM. After all, the on-board emissions analyzer on this car is the AFS and the downstream oxygen sensor, and they found that something was amiss with the converter.

The PIDs reflected that what the sensors were sensing was bad. Also, using bi-directional controls to make the fuel mixture richer and leaner, the sensors acted normally. So, how was it that the sensors were good, the emission analyzer measurements were good, but the PCM thinks the cat is bad? Perhaps an exhaust leak was throwing off the sensors, but a smoke machine found none.

I now performed a simple test that should have been performed after my initial test. I took a look at Enhanced OBDII data on my scan tool and looked up the proper voltage for AFS sensors on Nissans. Nissan AFS sensors should be at 1.47 volts. I measured the vehicle at 1.50 volts. So, the AFS sensor was not the problem, since it was in the proper range.

Checking Global OBDII for data and freeze frame is an important step in any drivability diagnosis.

Graphing the sensors told me the story and led me to the repair. There are two catalytic converters on this car. The downstream oxygen sensor on this vehicle was located after the first cat, but before the second cat. An exhaust gas analyzer would not find the problem while a functional HO2S would. That’s why the post-cat HO2S sensor is zigzagging when it should be straight—the front catalytic converter needs to be replaced. Nissan recommends replacing both should one fail, and they are sold only as a pair.

On to a 2004 Toyota Camry. On this car, I took advantage of the Catalyst Efficient test incorporated into the ATS EScan scan tool. The first one, tested at idle, passed with 99 percent efficiency. The second test was performed at cruising rpm and failed with a 42 percent efficiency. The lesson here is using freeze frame data to see when the fail occurred. I have repaired vehicles where the test failed at idle and passed at a higher RPM. Always take a look at the freeze frame data. In the case of this Toyota, freeze frame definitely revealed that the problem was off idle with recorded PIDs showing a 50 percent load on a warm engine running at 72 mph at 2,709 rpms.

The fix for this vehicle was to reprogram the PCM (per TSB, see how looking this stuff up pays off) and run a couple of cans of Run Rite fuel system cleaner through the system. Yes, I have had success restoring some life into catalytic converters using this method. The cleaner helped clean up the converter and got this vehicle back on the road. A word of caution, though. Sometimes, engines that are severely carboned will spew chunks of carbon out during the cleaning process, especially when done improperly. This can actually lead to blockage of the cat’s substrate and/or damage to it.

This Mode $06 screen shows questionable results. Aging oxygen sensors can provide slow feedback that may impact cat testing results.

Our next and last vehicle came in with another P0420 DTC that was caused by a vehicle that was in desperate need of maintenance. As previous stated there are ten top causes for this DTC, and this one could have been avoided if the owner had maintained their vehicle. The problem with this 2000 Honda Civic was that the ignition system was in very poor shape and caused the converter to load up with fuel causing it to deteriorate. After running a Catalytic Efficiency test and Mode 6 data it confirmed that this converter was really bad and most likely could not be saved by just performing a tune up and fuel system cleaning.

After the tune up was performed the engine was running good, I tested again but the results from both the efficiency and Mode 6 test were still bad. I appreciated being called in to check this vehicle out as it makes a good case study. I had the tech remove both the front and rear O2 sensors so I could install my video scope to take a look at the converter. The picture from the upstream O2 sensor revealed the substrate was in poor shape but the back O2 sensor revealed that it was over for this converter. This converter had to be replaced.

A Few Words Of Note
In many cases, repairing a catalytic efficiency code will require cat replacement. Before you do, though, take the time to inspect the condition of the old cat’s substrate. Blocked, broken, or melted substrate is a sure sign the cat didn’t fail of old age alone. It is vitally important that you check that “top ten” list and find the culprit responsible for its early demise.

The ATS EScan allows a bay test that mimics the ECM’s own catalytic efficiency monitor.

Another common mistake is choosing a cheap replacement cat. The problem is that when a poor quality converter is installed, the Malfunction Indicator Lamp (MIL) may soon return. That’s because the quality of the substrate may not be sufficient to meet the normal requirements of the engine and soon are unable to meet the PCM’s standards. Be sure to use a replacement from a quality aftermarket manufacturer or the OE part itself.

One last note on repairing P0420 and P0430 DTCs. Make sure you do all the paper work. It’s an EPA law that you must keep, hold and tag the converter for 15 days, provide the vehicle owner with the converter paper work and keep your own copy of that paperwork on hand for at least six months.

Diagnosing this common problem involves more than swapping converters. It takes the dedication of a professional tech to find out why the cat failed in the first place and a correction of that root cause to avoid a reoccurrence of the same complaint. Do your homework, research the service information, and verify that all other systems are working properly to insure that MIL stays off.

Subscribe to Motor Age and receive articles like this every month…absolutely free. Click here

Sponsored Recommendations

Cam & Crank Sensor Training Series

Today's advanced engines depend on accurate and consistent information from cam and crank sensors for performance, fuel economy and emissions, and not all cam and crank sensors...

Buying Your Next Diagnostic Tablet

When you last invested in a diagnostic tablet for your shop, did you consider it a potential gateway to increased revenue? Probably not. It’s likely you viewed it as a necessary...

Access Carside OEM Repair Data with MOTOR TruSpeed

Now available on all Autel MaxiSYS Ultra Series tools, MOTOR TruSpeed Repair delivers expanded OEM service and repair data within days of being published by

ADAS Case Study: From 10 Calibrations a Month to Over 10 A Day

Originally published by Vehicle Service Pros, March 26, 2024

Voice Your Opinion!

To join the conversation, and become an exclusive member of Vehicle Service Pros, create an account today!