What is code P0420 on Peugeot 308?

The OBD2 diagnostic trouble code P0420, “Catalyst System Efficiency Below Threshold Bank 1,” indicates that the Powertrain Control Module (PCM) has detected that the catalytic converter on Bank 1 is not functioning as efficiently as it should. Specifically on the Peugeot 308 (2007 and later models), this code is frequently triggered due to the vehicle’s reliance on a relatively small catalytic converter for emissions control, combined with the common occurrence of issues related to the oxygen sensors and exhaust leaks. The PCM monitors the readings from both the upstream (before the converter) and downstream (after the converter) oxygen sensors. If the downstream sensor’s readings are too similar to the upstream sensor’s readings, it suggests that the catalytic converter isn't effectively reducing harmful emissions, triggering the P0420 code. The Peugeot 308, particularly in its earlier iterations, has been known to experience issues with its exhaust system, leading to increased backpressure and reduced converter efficiency. The P0420 Peugeot 308 is a common diagnostic outcome, often following other related codes. The catalytic converter’s primary function is to reduce harmful emissions from the engine by converting pollutants like hydrocarbons, carbon monoxide, and nitrogen oxides into less harmful substances. It achieves this through chemical reactions facilitated by heat and a catalyst material. A failing catalytic converter will allow these pollutants to pass through the exhaust system, leading to increased emissions and triggering the P0420 code. While the code itself doesn’t always indicate a completely failed converter, it’s a strong indicator that there’s a problem needing attention. Ignoring this code can lead to further damage to the engine and potentially violate emissions regulations, particularly if the vehicle is subject to periodic inspections.

Symptoms of P0420 on Peugeot 308

  • Dashboard Check Engine Light illuminated – This is the most obvious symptom.
  • Slightly reduced engine power – The car may feel a little sluggish, especially during acceleration.
  • Increased fuel consumption – The engine may consume more fuel than usual, although this is often subtle.
  • Slight exhaust smell – A faint smell of unburnt fuel or sulfur may be noticeable, particularly after driving.
  • Rough idling – The engine might idle unevenly or vibrate slightly.
  • Hesitation during acceleration – The car may hesitate before fully accelerating.
  • Reduced throttle response – The accelerator pedal may not feel as responsive as it should.
  • Increased noise from the exhaust system – A rattling or hissing sound could indicate an exhaust leak contributing to the problem.

Causes of P0420 on Peugeot 308 – by frequency

  • Faulty Oxygen Sensors (Upstream & Downstream): The most common cause, particularly on the Peugeot 308. Sensor contamination or electrical issues can lead to inaccurate readings. Specific to the 2007-2010 models, a common issue is the wiring harness connector for the upstream sensor becoming brittle and prone to corrosion.
  • Exhaust Leak: Leaks upstream of the catalytic converter can introduce extra oxygen, skewing the sensor readings. Common leak points include manifold gaskets and exhaust pipe joints.
  • Failing Catalytic Converter: While not always the immediate cause, a degraded or damaged catalytic converter will eventually trigger the P0420 code.
  • Contaminated Catalytic Converter: Oil leaks or coolant leaks can contaminate the catalytic converter, reducing its efficiency.
  • PCM Software Issue: Rarely, a software glitch within the PCM can misinterpret sensor data.
  • Loose or Corroded Wiring Connections: Poor connections to the oxygen sensors or PCM can cause inaccurate readings.
  • Faulty Fuel Injectors: Leaky injectors can introduce unburnt fuel into the exhaust system, affecting converter efficiency.
  • Engine Oil Issues: Using the wrong viscosity oil or low oil levels can damage the catalytic converter over time.

Warning

Ignoring a P0420 code can lead to increased emissions, potential damage to the catalytic converter, and possible failure to pass emissions inspections. Furthermore, prolonged operation with a failing catalytic converter can negatively impact engine performance and longevity.

Mechanic Tip

A common mistake is immediately replacing the catalytic converter without diagnosing the root cause. Often, a faulty oxygen sensor or exhaust leak is the actual culprit. Thoroughly inspect the exhaust system and oxygen sensor wiring before replacing the converter – this saves time and money.

Step-by-step diagnostic procedure

  1. Scan for Codes: Use an OBD2 scanner to confirm the P0420 code and retrieve any related codes.
  2. Visual Inspection: Inspect the exhaust system for leaks, cracks, or damage. Pay close attention to the manifold gaskets, exhaust pipe joints, and catalytic converter.
  3. Oxygen Sensor Testing: Use a multimeter to check the resistance of the upstream and downstream oxygen sensors. Compare these readings to the manufacturer’s specifications.
  4. Oxygen Sensor Voltage Monitoring: Monitor the oxygen sensor voltage readings while the engine is running. Look for erratic or unusual fluctuations.
  5. Check Wiring and Connectors: Inspect the wiring and connectors for the oxygen sensors and PCM for corrosion, damage, or loose connections. Clean and secure any suspect connections.
  6. Backpressure Test: Perform a backpressure test to check for excessive exhaust restriction. This can indicate a clogged catalytic converter or exhaust system blockage. (Requires specialized equipment).
  7. Catalytic Converter Temperature Test: Use an infrared thermometer to measure the temperature of the catalytic converter inlet and outlet. A significant temperature difference indicates proper converter function.
  8. PCM Data Analysis: Utilize advanced scan tools to analyze PCM data, including sensor readings and fuel trim values, for anomalies.

Mechanic Tip

When testing oxygen sensors, ensure the engine is at operating temperature. Sensor readings will vary significantly at idle versus under load. Use a scan tool to monitor sensor data under different driving conditions.

Repair solutions and costs for Peugeot 308

Repair DIY Cost (EUR) Garage Cost (EUR) Difficulty Time
Oxygen Sensor Replacement (Upstream) 50-150 150-300 Medium 1-2 hours
Oxygen Sensor Replacement (Downstream) 60-180 180-350 Medium 1-2 hours
Exhaust Leak Repair (Manifold Gasket) 80-200 200-400 Medium 2-4 hours
Exhaust Pipe Repair/Replacement 150-400 300-600 Medium 3-6 hours
Catalytic Converter Replacement 300-800 500-1200 Hard 4-8 hours (including removal & installation)
PCM Software Calibration (if applicable) 0-200 200-400 Easy 30-60 minutes
Wiring Repair/Connector Cleaning 30-100 100-250 Easy 30-60 minutes

Can I drive with code P0420?

Driving with a P0420 code is generally permissible for short distances, but it’s not recommended for extended periods. While the vehicle might still run, the reduced fuel efficiency and potential for increased emissions are significant concerns. Prolonged driving with a failing catalytic converter can exacerbate the problem, potentially leading to further damage to the engine and increased repair costs. It's best to address the underlying issue as soon as possible to avoid complications. If the code is accompanied by other symptoms, such as rough idling or reduced power, it’s even more important to get the vehicle inspected and repaired promptly.

Frequently Asked Questions

Q: What is the repair cost for a P0420 code on a Peugeot 308?

A: The cost can vary greatly depending on the cause. Oxygen