Catalyst System Efficiency Below Threshold (Bank 2)


The PCM has detected a catalyst efficiency that is below the normal operating threshold for a specific engine bank. The PCM utilizes exhaust composition and temperature data from the oxygen sensors to calculate what degree of catalyst efficiency is required. If this level is not detected, then the code will be stored and the service engine light will be illuminated. “Bank 2” indicates that the condition causing the code to be stored is located in the catalytic converter for engine bank number two. Engine bank two is typically the bank of the engine which does not contain the number one cylinder. See the manufacturer’s service manual (or equivalent) for the location of the number one cylinder.

Code Set Parameters

The PCM uses input data received from the upstream and downstream oxygen sensors to calculate catalyst efficiency in each respective converter. Upstream oxygen sensor readings tend to fluctuate in conjunction with changing fuel mixture conditions. Downstream oxygen sensors tend to remain more steady then upstream sensors. If the individual sensor readings are too similar in response time or level, a code will be stored and a malfunction indicator lamp will be illuminated.


Symptoms may be as minor as only a stored code and an illuminated service engine soon lamp or as major as a no start or engine stall condition. If the catalytic converter is below the threshold for efficiency or if the oxygen sensor/s is faulty, there is likely to be no drivability issues. However, if the converter has broken or melted internal components, engine hesitation, an overall lack of engine performance, hissing noises when accelerating, or even a no start/engine stall condition may occur.

Common Causes

The most common cause of this code is due to a faulty catalytic converter. Bad oxygen sensors are also a possibility but if this is the case an oxygen sensor code will usually accompany the catalytic converter code. Always diagnose and repair oxygen sensor codes before attempting to diagnose catalytic converter codes. Since the catalytic converter is not designed to wear out, its failure is normally associated with some contributing malfunction. Contributing factors in catalytic converter failure may include incorrect fuel usage, excessive fuel being dumped into the exhaust system due to a faulty coolant temperature sensor, mass air flow sensor, manifold air pressure sensor, fuel pressure regulator, or fuel injection component, an ignition misfire, retarded spark timing, or oil contamination. Leaks from an exhaust manifold, down pipe, flex hose, or other exhaust component that is upstream from the catalytic converter can also appear to the PCM as catalytic converter failure.

Common Misdiagnosis

The most common misdiagnosis is caused by not thoroughly investigating what led to catalytic converter failure. Technicians report that repeated catalytic converter failure occurs when other codes are present and left unattended for long periods of time. Engine misfires are known to deteriorate the platinum element of the catalytic converter, as is excessively rich exhaust. The next most common misdiagnosis comes from oxygen sensor replacement. Oxygen sensor failure should be verified before replacement. Techs report that oxygen sensors are often replaced blindly in order to avoid costly catalytic converter replacement. This just leads to added expense. Also, aftermarket and “rebuilt” catalytic converters have proven problematic. Although they may cost much less, they provide neither the efficiency nor the longevity of OEM quality catalytic converters.


  • The catalytic converter is used exclusively to reduce exhaust emissions in gasoline burning vehicles with internal combustion engines
  • The catalytic converter is an in line device that resembles a muffler or resonator in exterior appearance although it differs greatly from either internally
  • The catalytic converter gets much hotter than a muffler when the engine is running and especially immediately after the vehicle has been driven
  • Retarded ignition timing, lean fuel conditions, and engine misfires can increase catalytic converter temperatures to dangerous levels
  • In some instances, the catalytic converter will reach temperatures that cause it to become “red hot” and present a high risk from fire if flammable liquids are leaked or spilled thereon
  • The catalytic converter uses a system of interwoven fibers (that contain a high concentrate of platinum) packed tightly into the metal housing to restrict and filter excessive noxious oxide fragments
  • The noxious oxide fragments (created by fuel that has not been sufficiently atomized) are then incinerated by the extreme temperatures (500 to 800-degrees Fahrenheit) found inside of the catalytic converter
  • Prior to beginning your diagnosis, verify that the catalytic converter is not under a manufacturer’s warranty
  • Catalytic converters typically carry a 100,000-mile federally mandated warranty, regardless of vehicle year model
  • If the catalytic converter is not under warranty, then begin by inspecting the exhaust system for leaks
  • If exhaust leaks are detected (particularly before the catalytic converter), repair them as necessary, reset the code, and retest the system. Several tools may be needed to successfully diagnose this code if no exhaust leaks are detected
  • A suitable scanner, a digital volt/ohmmeter, and a temperature gun will help to perform a thorough diagnosis
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • Proceed by raising the vehicle and pointing the temperature gun at the exhaust pipe before and after the catalytic converter in question (this is much easier if the temp gun is equipped with a laser pointer)
  • Compare your findings with manufacturer’s specifications
  • If your findings do not coincide with what the manufacturer recommends, then the catalytic converter is most likely bad
  • If your findings are in line with the manufacturer’s specifications, then use the scanner and oscilloscope to monitor upstream and downstream oxygen sensor operation on the affected engine bank
  • Start the engine, test drive the vehicle, then park it and allow the engine to idle
  • After the engine has reached normal operating temperature and the engine control system has entered closed loop operation, the upstream sensor should fluctuate rapidly from lean to rich (approximately .350 to .900 volts)
  • The downstream oxygen sensor should find a reading near center (about .500 volts) and it should hold this reading as long as the engine idles
  • If the downstream oxygen sensor fluctuates in a similar manner to the upstream oxygen sensor, the catalytic converter is most likely faulty. If either the upstream or downstream oxygen sensors are slow to respond, or fail to respond, to changing engine conditions the respective sensor may be faulty
  • However, if this is the case an oxygen sensor code should also accompany the catalytic converter code
  • Remember to diagnose and repair oxygen sensor codes, fuel trim codes, fuel mixture codes, or misfire codes first before attempting to diagnose a catalytic converter code.