What does this OBD II code mean?
This generic OBD2 code is set when the Powertrain Control Module (PCM) suspects of a deficient or faulty component in the catalyst system.
This particular code is pointing out the bank 1, in other words, the bank of cylinders containing cylinder #1.
Why does this OBD II code appear?
The powertrain control module (PCM) calculate the catalyst system efficiency using two sensors: the air-fuel ratio sensor (A/F sensor) located before the catalytic converter and the traditional oxygen sensor located after the catalytic converter. Those sensors are usually designated as the upstream oxygen sensor and the downstream oxygen sensor respectively. It’s not uncommon to find literature referring to them also as wideband oxygen sensor (upstream) and narrowband oxygen sensor (downstream). In order to determine the catalyst efficiency, the PCM first analyzes the upstream sensor data and then compares it with the downstream information. If both sensors are reporting similar results then the PCM assumes that the catalyst system is underperforming. That’s because the expected result is a “cleaner” exhaust reading after the catalytic converter.
The OBD2 code P0420 is stored in memory when the PCM detects a low efficiency in the catalyst system.
What symptoms may occur with this OBD II code?
Usually, the only symptom is the CEL lit. The Check Engine Light will turn on after few driving cycles to alert the driver about the situation. But under some circumstances, the CEL will blink repeatedly. That’s indicative of a serious condition with possible consequences to the engine.
Depending on the real root of the problem you may encounter other symptoms related to a clogged catalytic converter:
- Poor engine performance.
- Decreased fuel economy
- Increased engine coolant temperature (if the converter is highly clogged)
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Unusual exhaust fumes smell (rotten eggs)
What are the causes that trigger this OBD II code?
There're many factors that could cause a P0420 code including among others the following:
- Damaged, burnt, corroded, open or shorted O2 sensor wiring.
- Worn, corroded, burnt, disconnected or bent O2 sensor wiring harness connector pins.
- Faulty or deficient downstream oxygen sensor
- Leak between the exhaust pipe and the downstream O2 sensor.
- Faulty, deficient or clogged catalytic converter.
- Even when unlikely, a defective PCM cannot be discarded
Diagnostic strategy for troubleshooting this OBD II code
Warning: If any other code related to the upstream oxygen sensor or engine misfires are present then you must start diagnosing and solving those codes first.
It's highly recommended starting the diagnostic process with the usual information gathering:
- With the help of a professional OBD2 scan tool, preferably one with OEM capabilities, take note of all data trouble codes in memory, including pending codes.
- Take note of freeze frame information (if present).
- Query the vehicle owner about what, when and where the failure occurs. Does it happen with the engine hot? Cold? Under load? All the standard questions.
- Take note of the vehicle VIN and find a reliable source of technical reference.
Once you are done, the next step is discarding an intermittent condition.
- Using your scan tool clear DTC memory.
- Perform a complete driving cycle according to OEM procedures.
If the code P0420 disappears then inform the owner about the intermittent nature of the code and explain that further testing with the code present will be required. If the code does appear then continue to the testing stage, consisting in discarding the possible causes of the problem.
IMPORTANT: the diagnostic procedure explained in this section should be taken as a guideline. Always refer to specific OEM procedures when possible.
The workflow for the diagnostic process is:
- The tests should be executed in sequential order, in other words, complete step 1 before going to step 2 and so on.
- Repair, fix, or replace any component if you suspect is not in good condition.
- If any repair, fix, reprogramming or component replacement is made during any step then you should clear DTC memory and repeat the whole KOER driving cycle.
- If the DTC appear again then you should continue to the next step.
- If the DTC does not appear again after completing the KOER tests then you can assume the problem is fixed, in that case, continuing with the remaining steps would be optional.
The diagnostic process for the code P0420 is as follows:
- Sensor harness wiring and connector: carry out an exhaustive visual inspection of bank 1 downstream oxygen sensor wiring and connectors. Pay special attention to burnt, corroded, open, shorted to ground or deteriorated wiring and/or connector. Unplug the sensor and check for corrosion, damage, bent pins, etc.
- Electrical tests: with the help of a digital volt-ohm meter (DVOM) and the adequate wiring diagram perform a KOEO check on the downstream O2 sensor. Verify supply and/or reference voltage, electrical resistance, continuity, current and ground integrity. Compare the obtained values with the appropriate OEM literature.
- Exhaust system leak: any leak between the exhaust pipe and the downstream oxygen sensor could cause an incorrect reading of the exhaust gases. Inspect the bank #1 exhaust and repair any leak you may find.
- Downstream oxygen sensor: most vehicle manufacturers use traditional oxygen sensors (instead of A/F sensors) for the post-catalyst monitoring. This opens you the possibility of performing the sensor test using a DVOM, an oscilloscope or a scan tool. Refer to the proper OEM literature in order to decide what is the best approach. Replace the O2 sensor if necessary and repeat all KOER tests before continuing.
- Catalytic converter test: start the engine and let it warm up until it reaches its normal operating temperature. Increase and maintain the engine RPM in the 2000-2500 range and then use a digital automotive pyrometer to take note of the inlet temperature and outlet temperature of the catalytic converter. Under normal conditions, the inlet temperature should be approximately 150°C hotter than outlet temperature. If both temperatures are similar then replace the catalytic converter.
- Catalytic converter alternative tests: using a pyrometer is arguably the easier way to test the catalytic converter condition but not the only one. With the help of a professional scan tool put on the display the live data of the upstream and downstream O2 sensors. If the downstream oxygen sensor mimics the upstream sensor readings then replace the catalytic converter. Another method to diagnose a clogged catalytic converter is doing a vacuum reading in the intake manifold. If the converter is obstructed will decrease the engine vacuum available.
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Problems with the powertrain control module: the most unlikely cause for this code is a faulty PCM or a corrupted program. Yet, if you are completely certain that all prior tests passed then you must remove the PCM and check it, reprogram it or replace it as necessary.
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