P0423Generic code

P0423 Code: Heated Catalyst Efficiency Below Threshold (Bank 1)

The P0423 code means the electrically heated catalytic converter on bank 1 is not reaching the efficiency it is required to have. Rule out exhaust leaks, slow O2 sensors, mixture and misfire problems, and the heater itself before you replace the converter.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$100 to $2,500
Difficulty
Medium
System
Emissions Control
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0423 code means

Some low-emission vehicles carry a converter with an electric heater so it works from the moment the engine starts. The P0423 code says its efficiency is below the limit.

The computer compares the signal from the upstream sensor with the one from the downstream sensor. If the downstream signal starts to copy the upstream one, the converter is no longer storing oxygen. Before you replace it, rule out exhaust leaks, slow sensors and problems with the mixture or with misfires.

The diagnosis is the same as for the P0420 code, with the heater added to the list of things to check.

Before you replace a converter, check the downstream sensor with a simple test. With the engine hot, rev it hard with the vehicle parked, and the sensor should rise above 0.8 volts. Then let off the throttle, and it should drop below 0.2 volts. If it responds that way, the sensor is healthy and the converter is probably worn out. Remember that a new converter will be ruined the same way if you do not fix the oil consumption or the misfires that damaged the old one.

P0423 code symptoms

  • Check engine light on.
  • Usually no driving symptoms.
  • Sulfur (rotten egg) smell from the exhaust.
  • Fails the emissions inspection.

Likely causes of the P0423 code

  1. Converter worn out or contaminated by oil, coolant or misfires.
  2. Exhaust leak ahead of the downstream sensor.
  3. Slow oxygen sensor. An aging sensor can make a healthy converter look worn out.
  4. Mixture out of range. Fuel trims beyond plus or minus 10 percent upset the converter test.
  5. Faulty converter heater on heated converters.

How to diagnose the P0423 code step by step

  1. Read the upstream and downstream sensors with the engine hot.
  2. Look for exhaust leaks.
  3. Check the fuel trims and look for misfires.
  4. Measure the temperature before and after the converter with an infrared thermometer.
  5. Check the heater for resistance and supply.
  6. Test the downstream sensor by revving hard (above 0.8 volts) and letting off (below 0.2 volts).

How to fix the P0423 code

  • Fix leaks, sensors, mixture or misfires first.
  • Replace the converter with one approved for your vehicle.
  • Repair the heater if that is the case.
  • Clear the codes and complete the drive cycle.

Technical reference data

  • Downstream sensor with a healthy converter: nearly steady between 0.5 and 0.8 volts.
  • Converter working temperature: 400 to 800 °C (about 750 to 1,470 °F).
  • These are approximate values, so check the manufacturer data for your engine.

Brand-specific notes

Electrically heated converters are rare. Separate main converters are common on V6 and V8 engines.

P0423 on BMW

Electrically heated catalytic converters show up on some BMW vehicles.

P0423 on Volvo

Electrically heated catalytic converters show up on some Volvo vehicles.

P0423 on V6 and V8 Engines

Separate main converters are common on V6 and V8 engines.

Frequently asked questions about P0423

Can I drive with a P0423 code?
Yes. It normally does not affect driving, but you will not pass an emissions inspection.
How is it different from P0420?
It refers to a specific converter on vehicles that have more than one. The diagnosis follows the same logic.
Do I have to replace the catalytic converter?
Only after you rule out the causes that skew the test, such as exhaust leaks, a slow sensor, a bad mixture or misfires.
How much does it cost to fix P0423?
Between $100 and $2,500, depending on the converter. Diagnosing the P0423 code properly first keeps you from paying for parts you do not need.
What does the P0423 code mean?
It is a generic OBD2 code for heated catalyst efficiency below threshold on bank 1. The converter is not reaching the efficiency the computer requires.
Will a P0423 code clear itself?
Not for good. If the converter is worn out or the heater is faulty, the code will return.

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