Check engine light pops on while driving, code P0420

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

The suspect is the driver's side catalytic converter needs to be replaced. Don't be mislead by a lot of people telling you to replace oxygen sensors. It takes properly-working front and rear O2 sensors to detect when no change is taking place in the exhaust gas as it passes through the catalytic converter.

I can't read the listings. That's a common problem, so I just go by the fault code description.

I do have to correct myself on the list of suspects. It is important to understand that fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a sensor or other part is referenced in a fault code, it is actually the cause of that code about half of the time. With a code 420, I was taught it can only be caused by the catalytic converter, and it can only be detected with two properly-working oxygen sensors, one ahead of the converter and one after it. There is always a list of things to check first before spending our customer's money on a part referenced in a fault code, but as I mentioned, only the converter can cause this one, ... Or so I thought. In fact, it can be set by a defective sensor or a wiring problem for one of them. That takes care of the one mistake I allow myself per year!

I'm going to post the list of troubleshooting steps so you can see what we are supposed to go through before blindly replacing a part, but to do it justice, I have to copy and format some drawings. I'll work on that at home tonight, to put it in a format I can upload and you can read. I'm pretty confident the converter is going to have to be replaced, but look through the list first. You might recognize something on it that pertains to your car.

There's always a long list of conditions that have to be met for a fault code to set, and one of those conditions is that certain other fault codes aren't already set. Many things are compared to each other to figure out when there's a problem, but if one of those things has been detected as having a problem, it can't be relied on for use as a reference, so anything else that is compared to the defective circuit can't be tested, and no fault code will set. Does that make sense?

To say that a different way, it takes multiple properly-operating circuits for the computer to be able to detect when there's a defect. This is where the logic comes from that says setting code 420 requires the oxygen sensor circuits to be working properly.

Okay, here's what Chrysler lists to do before condemning the catalytic converter: (After your eyes glaze over, look at the second to last sentence).

P0420-CATALYST EFFICIENCY (BANK 1)

Drawing 1

Diagram 2

For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

The State of Change (SOC) catalyst monitor uses the signals from both the Upstream and Downstream O2 Sensors to detect aging of the catalyst. Based on the fact that when a catalyst ages, it loses some of its Oxygen Storage Capacity (OSC). As a result, part of the untreated exhaust gases can breakthrough the catalyst and causes the Downstream O2 Sensor to deviate from its neutral (Stoichiometric) position. By observing the activities in the Downstream O2 Sensor signal, the degradation level of catalyst can be detected. In general, the higher the Downstream O2 Sensor SOC value, the more exhaust gas breakthrough and the lower the OSC of the Catalytic Converter.

- When Monitored:
The monitor will run at between 1400 and 2300 RPM and MAP vacuum between 40 to 70 kPa (15.0 and 21.0 (Hg)).

- Set Condition:
If the final State of Change index is within the calibrated fail threshold. Two trip fault. Three good trips to turn off the MIL.

Drawing 3

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. See: Computers and Control Systems > Initial Inspection and Diagnostic Overview > Pre-Diagnostic Troubleshooting Procedure.

1. ACTIVE DIAGNOSTIC TROUBLE CODE (DTC)

NOTE: A new rear O2 Sensor along with an aging front O2 Sensor may cause the DTC to set. Review the repair history of the vehicle before continuing.

NOTE: If an O2 Sensor DTC set along with the Catalytic Converter Efficiency DTC diagnose the O2 Sensor DTCs before continuing.

NOTE: Check for contaminants that may have damaged the O2 Sensor and Catalytic Converter: contaminated fuel, unapproved silicone, oil and coolant, repair necessary.

1. Start the engine.
2. Allow the engine to reach normal operating temperature.
3. With the scan tool, read DTCs.

NOTE: It may be necessary to drive the vehicle to meet the conditions to set this DTC, try to repeat the conditions in which the fault originally set by reviewing the Freeze Frame data.

Is the DTC Active or Pending at this time?

Yes

- Go To 2

No

- Perform the INTERMITTENT CONDITION diagnostic procedure. See: Computers and Control Systems > Initial Inspection and Diagnostic Overview > Intermittent Condition Test.

2. VISUALLY INSPECT CATALYTIC CONVERTER
1. Inspect the Catalytic Converter for the following damage:
- Damage Catalytic Converter, dents or holes.
- Severe discoloration caused by overheating the Catalytic Converter.
- Catalytic Converter broke internally.
- Inspect both ends of the converter, inlet and outlet.
- Leaking Catalytic Converter.

Were any problems found?

Yes

- Replace the Catalytic Converter. Repair the condition that may have caused the failure. See: Catalytic Converter > Removal and Replacement > Catalytic Converter - Removal.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 3

3. (K902) O2 SENSOR 1/1 RETURN CIRCUIT VOLTAGE CHECK

Drawing 4

1. Turn the ignition off.
2. Disconnect the O2 Sensor 1/1 harness connector.
3. Turn the ignition on.
4. Measure the voltage on the (K902) O2 Sensor 1/1 Return circuit in the O2 Sensor 1/1 harness connector.

Is the voltage at 2.5 Volts?

Yes

- Go To 4

No

- Go To 5

4. (K904) O2 SENSOR 1/2 RETURN CIRCUIT VOLTAGE CHECK

Drawing 5

1. Turn the ignition off.
2. Disconnect the O2 Sensor 1/2 harness connector.
3. Turn the ignition on.
4. Measure the voltage on the (K904) O2 Sensor 1/2 Return circuit in the O2 Sensor 1/2 harness connector.

Is the voltage at 2.5 Volts?

Yes

- Go To 10

No

- Go To 7

5. (K902) O2 SENSOR 1/1 RETURN CIRCUIT SHORTED TO GROUND

Drawing 6

1. Turn the ignition off.
2. Disconnect the PCM C2 harness connector.
3. Measure the resistance between ground and the (K902) O2 Sensor 1/1 Return circuit in the O2 Sensor 1/1 harness connector.

Is the resistance below 100 Ohms?

Yes

- Repair the short to ground in the (K902) O2 Sensor 1/1 Return circuit.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 6

6. (K902) O2 SENSOR 1/1 RETURN CIRCUIT OPEN

Drawing 7

1. Measure the resistance of the (K902) O2 Sensor 1/1 Return circuit from the O2 Sensor 1/1 harness connector to the PCM C2 harness connector.

Is the resistance below 5.0 Ohms?

Yes

- Go To 9

No

- Repair the open in the (K902) O2 Return Upstream circuit.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

7. (K904) O2 SENSOR 1/2 RETURN CIRCUIT SHORTED TO GROUND

Drawing 8

1. Turn the ignition off.
2. Disconnect the O2 Sensor 1/2 harness connector.
3. Measure the resistance between ground and the (K904) O2 Sensor 1/2 Return circuit in the O2 Sensor 1/2 harness connector.

Is the resistance below 100 Ohms?

Yes

- Repair the short to ground in the (K904) O2 Sensor 1/2 Return circuit.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 8

8. (K904) O2 SENSOR 1/2 RETURN CIRCUIT OPEN

Drawing 9

1. Measure the resistance of the (K904) O2 Sensor 1/2 Return circuit from the O2 Sensor 1/2 harness connector to the PCM C2 harness connector.

Is the resistance below 5.0 Ohms?

Yes

- Go To 9

No

- Repair the open in the (K904) O2 Sensor 1/2 Return circuit.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

9. POWERTRAIN CONTROL MODULE (PCM)
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Sensor and the Powertrain Control Module (PCM).
- Look for any chafed, pierced, pinched or partially broken wires.
- Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the Sensor and the Powertrain Control Module connectors.
- Perform any Technical Service Bulletins that may apply.

Were there any problems found?

Yes

- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Replace and program the Powertrain Control Module. See: Engine Control Module > Removal and Replacement > Powertrain Control Module - Removal.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

10. CHECKING THE EXHAUST SYSTEM FOR LEAKS
1. Turn the ignition off.
2. Raise the vehicle.

WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near the catalytic converter. The temperature of the converter rises to a high level after a short period of engine operating time.

3. Connect Exhaust Cone or to Air Pressure Regulator (with hose) W-18-MIL-1146AS.

CAUTION: The air pressure must not exceed 27.6 kPa (4 psi), otherwise engine damage can occur.

4. Attach shop air to the air pressure regulator.
5. Adjust the Air Pressure Regulator to 27.6 kPa (4 psi).
6. Insert the exhaust cone into the vehicle tail pipe.
7. If the vehicle is equipped with dual exhaust. Use the with equipped attached plug, plug one side of the dual exhaust pipe. Pressurize the other as described above.
8. Apply Mopar(R) Air Leak Detector PN# 05191804AA (or an equivalent leak finder liquid) to the following areas:
- All welded joints from the exhaust manifold to 152.4 mm (6 inches) behind the downstream O2 Sensor
- O2 Sensor seal points
- O2 Sensor boss welds
- Flange/joint connection(s)
- Exhaust manifold to cylinder head connection(s)
- EGR solenoid gasket base and tube seal points (if equipped)

9. Watch for the liquid/soapy water to bubble.
10. Use the following definitions to help determine if system or component repair/replacement is necessary:
- Type 1 Leak is defined as a leak where very small foam like bubbles 1 mm (0.04 of an inch) or less appear. Any Type 1 or greater leaks found in welded joints, O2 Sensor seal points or O2 Sensor boss welds must be repaired or the component must be replaced.
- Type 2 Leak is defined as a leak where larger bubbles pea size, 8 mm (0.3 of an inch) or greater appear. Any Type 2 or greater leaks found in flange or joint connections, exhaust manifold to cylinder head connections or EGR gasket and tube seal points must be repaired or the components must be replaced.

Drawing 10

11. If a leak is found that matches the above definition, repair or replace the component as necessary.
12. Once the repair is complete, repeat the procedure to verify that all leaks have been repaired.

Were any exhaust leaks found?

Yes

- Repair or replace the leaking exhaust parts as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 11

11. ENGINE MECHANICAL CONDITION
1. Check the exhaust for excessive smoke caused by an internal problem in the engine.

Is an engine mechanical condition present?

Yes

- Repair the engine mechanical condition as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 12

12. AGING O2 SENSOR
1. A new Downstream O2 Sensor along with an aging Upstream O2 Sensor may cause the DTC to set.
2. Review the vehicles repair history.

Has the Downstream O2 Sensor been replaced without replacing the Upstream O2 Sensor?

Yes

- Replace the front O2 Sensor as necessary. See: Catalytic Converter > Removal and Replacement > Catalytic Converter - Removal.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.

No

- Go To 13

13. CATALYTIC CONVERTER

If there are no possible cause remaining, view repair.

Repair

- Replace the Catalytic Converter. See: Catalytic Converter > Removal and Replacement > Catalytic Converter - Removal.
- Perform the POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests > Powertrain Verification Test.