P0420 – Catalyst System Efficiency Below Threshold Bank 1 (Ford)

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By Stephen Darby (Contact Me)
Last Updated 2022-04-01
ASE Master Tech
CodeFault LocationProbable Cause
P0420 Catalyst System Efficiency Below Threshold Bank 1
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Catalytic converter, wiring, HO2S 2

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Table of Contents

  1. What Does Code P0420 Mean?
  2. What are the common causes of code P0420?
  3. What are the symptoms of code P0420?
  4. How do you troubleshoot code P0420?
  5. Get Help with P0420

What Does Code P0420 Mean?

If your Ford vehicle has stored a code P0420, it means that the powertrain control module (PCM) has detected a discrepancy in the catalyst efficiency for the upstream catalytic converter of engine bank one. Bank 1 specifies the bank of the engine which contains the number one cylinder. Code P0420 may be stored as the result of either a mechanical failure or an electronic issue.

Ford vehicles use a system of catalytic converters (typically two per engine bank) to help reduce engine emissions before they exit the exhaust. Catalytic converters are comprised of a steel housing containing a rectangular reduction element of platinum (and rhodium) enriched filtration material designed with a flow-through honeycomb pattern. Harmful nitrogen oxides are trapped inside the reduction element where high temperature helps to burn them away. Heated exhaust gas oxygen (HO2S) sensors are used to monitor exhaust oxygen content and catalytic converter efficiency. The PCM compares data from multiple HO2S sensors (before and aft of each upstream catalytic converter) to determine the degree of catalytic converter efficiency in your Ford vehicle. With the engine at normal operating temperature and the PCM in closed loop operation mode, the upstream HO2S (between the engine and the catalytic converter) should cycle constantly between 1 mV and 9 mV while downstream HO2S (between the catalytic converter and the muffler) should remain more stable.

When the PCM detects a discrepancy between the upstream HO2S and the downstream HO2S, indicating that catalyst system efficiency may be below the maximum allowable threshold, a code P0420 will be stored and a malfunction indicator lamp (MIL) may be illuminated. Multiple failures may be required for MIL illumination.

What are the common causes of code P0420?

  • Defective HO2S sensor
  • Blown HO2S fuse
  • Bad catalytic converter
  • Faulty fuel pressure regulator
  • Manifold air pressure or mass airflow sensor failure
  • Severe engine misfire

What are the symptoms of code P0420?

  • Decreased engine performance
  • Diminished fuel efficiency
  • Black smoke from exhaust
  • There may be no noticeable symptoms associated with this code

How do you troubleshoot code P0420?

I would begin diagnosing a stored code P0420 in my Ford by obtaining access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer (preferably with laser pointer) and a source of reliable vehicle information.

Search applicable Ford technical service bulletins (TSB) that replicate the code stored, vehicle (year, make, model, and engine), and symptoms exhibited. This information may be found in your vehicle information source. You may dramatically reduce diagnostic time and effort by locating the correct TSB.

As a prerequisite to diagnosis and repair of the P0420, one must always determine two basic things. First, use a scanner to observe HO2S activity (after the engine has reached normal operating temperature and the PCM has entered closed loop operation). By observing live HO2S data, you can determine if the catalyst is truly below its predetermined threshold or if the HO2S is malfunctioning. If the catalytic converter proves to be performing below its threshold, it is important to determine whether or not catalytic converter failure has been caused by extenuating circumstances. Conditions such as ignition misfires or an improper air-to-fuel mixture may have contributed to catalytic converter failure. Premature catalytic converter failure will be the likely result if misfires and fuel malfunctions are not rectified in prior to replacement. Obviously, engine misfire and fuel delivery related codes should be diagnosed and repaired before attempting to repair a P0420. I find that this is a good time for me to perform a visual inspection of related wiring and connectors. I am looking for wiring that has been cut, burned, or damaged.

I also like to use an infrared thermometer to test catalytic converter operation. I have found, when the catalytic converter is functioning efficiently, exhaust exiting the converter will be substantially hotter than exhaust entering the converter. Use caution when testing hot exhaust components.

Engine misfire codes can frequently be attributed to a defective ignition coil, spark plug, spark plug cable, primary wiring or PCM. In order to diagnose fuel delivery related codes, check fuel pressure and volume. Make sure that both are within specifications and the fuel is in good condition. Make sure that no engine vacuum leaks are present in the intake manifold and throttle body areas. Don’t forget to check the PCV and EGR for vacuum leaks, if a lean exhaust condition has been detected.

Before beginning a physical diagnosis, connect the scanner to the vehicle diagnostic port and retrieve all stored codes and pertinent freeze frame data. I like to write this information down just in case the code proves to be an intermittent one. Afterward, I clear the codes and test drive the vehicle until the code is restored or the PCM enters readiness mode.

If the P0420 is restored, utilize your source of vehicle information to obtain connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts (pertaining to the code and vehicle in question).

You may use the scanner to observe HO2S sensor data (recommended method) or use the DVOM and the appropriate wiring diagram to obtain “live data”. If you are going to use the scanner, narrow the data display field to include only HO2S information. This will yield a much more accurate reading. After the engine has reached normal operating temperature and the PCM has entered closed loop operation, compare upstream HO2S operation voltage against downstream HO2S sensor operation voltage. Pay particular attention to sensor cycling frequency. The upstream HO2S should cycle rapidly from lean to rich (1 mV to 9 mV) while the downstream HO2S locates a stable position between the two points. If the downstream HO2S mirrors the upstream HO2S, suspect that the catalytic converter is defective. If either of the two HO2S sensors behaves erratically, or fails to register, suspect either a failed sensor or a blown HO2S fuse. Return to your vehicle information source to locate applicable fuses, relays and other power sources for HO2S sensors.

Suspect PCM failure as a last resort. High mileage vehicles are more subject to catalytic converter failure.

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