I have Oxygen sensor replaced already, they are all new and check engine light is not on. I cannot pass the smog test, because monitors on catalyst, evaporative system, secondary air system and oxygen sensor are not ready This 1997 Jaguar xj6 has never given me any problem. The dealership - Cole European I bought the car said that it may be the problem in the computer, they can replace it, but they cannot guarantee it will solve the problem. The cost of replacing it will be $5,140. What an irresponsible answer to their customer.

The diagnostic trouble code (DTC) P0420 "Catalyst System Efficiency Below Threshold" is one possible outcome following the Catalytic Converter Monitor (test). This monitor is one of the last tests to be performed by the powertrain control module (PCM) because other failures in the system could compromise this test's result. Catalyst efficiency is tested by observing the oxygen sensor signals both before and after the catalyst. Before the catalyst, the signal should appear as a sine wave as it switches back and forth between rich (0.9 volts) and lean (0.1 volts). After the catalyst, the signal should be flat-lined on the rich side (about 0.7 volts). This is because a healthy catalyst will store oxygen and therefore remove the "lean" portion of the signal/waveform. If the signal after the catalyst still appears as a sine wave, the PCM concludes that the catalyst can no longer store oxygen and should therefore be replaced. In your specific scenario where a downstream (post-cat) O2 sensor was just replaced, I would theorize that this sensor had failed and was flat-lined. Then when it was replaced two things happened: 1.) the oxygen sensor began to work properly which allowed the oxygen sensor monitor (test) to complete, and 2.) the catalyst monitor then ran and used the new (accurate) sensor data to determine that the catalyst efficiency was below threshold. You can read more here: https://www.yourmechanic.com/article/p0420-obd-ii-trouble-code-catalyst-system-efficiency-below-threshold-bank-1-by-jamahl-walker

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This is common in the Nissan's could be the cam and crank sensor these go at around 70000 to 120000 should do both at once to be safe. that will fail an emissions test for sure. hope this helps

Not sure how old the vehicle is, or model, or engine, so I can only give general suggestions. You may need new catyitic converters, or O2 sensors. They sometimes can't be corrected, just replaced.

Catalyst System Efficiency Below Threshold I have put together the following information about the P0420 fault code. I have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.
OBD II Fault Code

• OBD II P0420

Fault Code Definition

• Catalyst System Efficiency Below Threshold

Symptoms

• Check Engine Light will illuminate
• In most cases, there are no adverse conditions noticed by the driver
• In some cases, there may be some performance problems noticed by the driver such as a lack of power from a restricted and/or damaged Catalytic Converter

Common Problems That Trigger the P0420 Code

• Inefficient Catalytic Converter(s)
• Defective Front or Rear Oxygen Sensor(s)
• Misfiring engines

Common Misdiagnoses

• Oxygen Sensors

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
• NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

• Most of the aftermarket Converter companies are re-working their designs so they can be certified for use in OBD-II vehicles in California

The Basics
The Catalytic Converter looks like a muffler. It is typically a stainless steel housing over a ceramic honeycomb core. The catalyst itself is made of platinum, palladium, or rhodium, all rare metals, which is why Catalytic Converters are so expensive. These elements reduce the toxicity of harmful exhaust gases that are expelled from the tail pipe. Catalytic converters are quite efficient, but if engine maintenance is neglected or an engine is allowed to "run rough," damage could occur, resulting in costly repairs. To replace the Catalytic Converter, the vehicle is raised to gain access to its underside. The converter is removed from the exhaust system and the new Catalytic Converter is installed.
Want to Learn More?
The Catalytic Converter is a sophisticated after-burning device designed to complete combustion of the exhaust gases that pass through it. It is a stainless steel container with an inlet and outlet pipe that looks similar to a muffler. Inside, the Catalytic Converter is a ceramic monolithic structure that has honeycomb-like passages running though it. This structure has several sections called beds that are thinly coated with rare metals, which react with the compounds in the exhaust gases to complete the combustion process, thereby cleaning the exhaust of harmful emissions.

• The first section of the Catalytic Converter is called the reduction bed and is coated with rhodium. It is called the reduction bed because its purpose is to reduce the NOx gases back into harmless nitrogen and oxygen.
• The next section of the Catalytic Converter is the oxygen storage bed, which is coated with cerium. Its purpose is to maintain an ideal level of oxygen for use by the rear of the converter. It does this by storing and releasing the oxygen that gets released from the reduction of NOx in the previous reduction bed.
• The oxygen is then available for use in the final oxidizing bed, which is coated with platinum and palladium. The purpose of the oxidizing bed is to complete the combustion of CO by adding oxygen. The oxidizing bed also uses oxygen to burn any of the raw HCs that still remain in the exhaust gases.

P0420 Diagnostic Theory for Shops and Technicians
The P0420 code is set when the Catalyst monitor sees a decrease in voltage from the rear Monitoring Oxygen Sensor(s) and an increase in switching activity-from rich to lean to rich, etc.-that closely resembles the front Oxygen Sensor(s) during the time the computer is activating the Catalytic Converter monitor test. The voltage threshold is usually a minimum of 650 millivolts, which indicates a low level of oxygen. When the voltage goes too far below the 650 millivolt minimum, it indicates a higher level of oxygen. This means that not all of the oxygen is being consumed by the combustion process or by the afterburning effect of the Catalytic Converter. When the oxygen level gets too high, it means that the Cerium or Oxygen storage bed has degraded to the point where it is no longer able to store oxygen created by the reduction of NOx (nitrogen and oxygen). This oxygen is essential for the rear Oxidation bed to complete the conversion of CO into CO2 and HCs into H20 and CO2.
Common Tests for Diagnosis of the P0420 Code

• Retrieve the code and write down the freeze frame information to be used as a baseline to test and verify any repair.
• If there are any misfires, ignition, fuel and/or intake problems, these must be repaired before the Catalyst code is addressed. Any misfire, ignition, and/or fuel system problem will quickly ruin a Catalyst. They are often the cause for code in the first place.
• Test drive the vehicle at or near the freeze frame conditions to verify that the rear Catalyst Monitoring Oxygen sensor is either mirroring the front Oxygen sensor and/or is not reaching the 650 millivolt threshold during 55-60 MPH cruise conditions. If either of theses conditions can be easily verified, then the Catalytic Converter is defective.
• If there is any doubt about the condition of the front and/or rear Oxygen sensors, then check the Mode 6 data for all the Oxygen sensor monitor tests. If any of the front or rear Oxygen sensors barely pass their Mode 6 tests, then clear all codes and perform a drive cycle to see how well the front and rear Oxygen sensors pass their Mode 6 tests. They need to pass the Mode 6 tests with flying colors or they will confuse the OBD-II diagnostic software and possibly trigger a false code P0420. This is an important concept because if the front Oxygen sensor is slow and barely passes its monitor tests, it can fool the computer into thinking that the Catalyst has failed because the computer just watches how closely the switching speed of the front Oxygen sensor compares to the switching speed of the rear Monitoring Oxygen sensor. If the rear Monitoring Oxygen sensor is losing bandwidth and not able to easily reach the 650 milli volt threshold-but still barely passes its monitor tests-then it, too, can fool the computer into setting a P0420 code.
• If the Catalyst has failed, be sure to check for any software updates for the power-train computer. Many of the OBD II-equipped vehicles require software updates whenever the Catalyst is replaced

As for your codes p1131 and 1151: Ford - Lack Of Heated Oxygen Sensor Bank 1 Sensor 1 Switches - Sensor Indicates Lean


Last week I had the very same catalyst, evap and ECG codes come up on my 1999 Mercury Mystique which is like a Ford Taurus. A friend was able to find a hole or leak in the hose connected, replaced it and it runs fine. I havent run it a full and straight 50 miles yet to clear the check engine light but that repair alone changed the entire way the car was poorly driving. Im sure that hasnt fixed all the probs but thats a good starting point.

My first thought would be the converter considering the codes....which will cause the sensors/heaters and other components to fail or read as being bad... I would call a dealer and make sure there are no recalls on the emissions systems and anything to do with the vehicle..
Also it would be worth your while to scan it with a TECH 2 scanner......You may have one I dont know......But I worked for Buick/Pontiac/GMC for 5 years and TECH 2 was a life saver...
It will tell you everything !! If it is running fine though I would just change the oxygen sensor,
clear the codes,test drive it then check it again.....But if you have no problem with it and hooked it to an emissions analyzer and it showed ok I wouldnt worry....

CODE P0421. DEFINITION-CATALYST EFFICIENCY LOW-BANK 1. EXPLANATION-THE OXYGEN SENSORS MONITOR THE CATALYTIC CONVERTERS ABILITY TO STORE OXYGEN. PROBLEM CAUSES- 1. CATALYTIC CONVERTER DEFECTIVE (FAILURE POSSIBLY DUE TO #2,3, OR 4) 2. ENGINE MISFIRE OR RUNNING CONDITION. 3. LARGE VACCUM LEAK 4. ENGINE OIL LEAKAGE INTO EXHAUST-VALVE GUIDE SEALS, PISTON RINGS. THE PCM HAS DETERMINED THAT THE WARM UP CATALYST EFFICIENCY FOR BANK 1 IS BELOW TRESHOLD FOR THE CURRENT ENGINE OPERATING CONDITION. (BANK 1 IDENTIFIES THE LOCATION OF CYLINDER #1, WHILE BANK 2 IDENTIFIES THE CYLINDERS ON THE OPPOSITE BANK).

replace O2 sensors, or your Catalytic Converter.

No this is not right. If you remove the battery everything resets anyways. If they are talking about relearn procedure, it should only require 50 miles. But if the light is on than either they did something wrong, created another problem, or there was already another one existing. Have the code read and let me know what it is. I'm in Ca too and used to prep european cars for smog, so I do know what I'm looking for. How did the rest do? What were the readings(if possible)?

the best way to reset it is with a scan tool.
disconnecting the battery for one minute will also remove the code.BUT
if your smog test uses an OBD scan as part of the process,then there will be a problem,because the monitors will not have been set,as they are reset if the battery is disconnected..
most tests allow only one monitor missing,and not the catalyst .
A drive cycle will have to be done to set the monitors.
Typically cold start coolant temp less than 50F,idle for 2 minutes no air condition, accelerate at half throttle to 50mph then drive 3 minutes with a/c on at 50mph steady.decelerate to 10mph without using brakes..Accelerate to 60mph and drive for 5 minutes at steady speed without a/c
deccelerate without braking and drive for 1 minute at 40mph.
stop the car ,use braking.
Shut off the car and add 4 gallons of gas.do not shut off the car for more than 5 minutes.
drive in 3rd gear or overdrive off with rpm 3000 for 1 minute.
decelerate to 10mph.without braking
accelerate to 40 mph in 4th gear.,or overdrive on
decelerate without braking to 10mph.
test complete.
this will set the o2 sensor heater ,o2 sensor,catalyst,evap purge.and the other monitors.