Direct Ozone Reduction Catalyst Efficiency Below Threshold


The PCM has detected a less than expected degree of direct ozone reduction catalyst efficiency.

Code Set Parameters

The PCM uses input data received from the upstream and downstream oxygen sensors to calculate the degree of direct ozone reduction catalyst efficiency for each respective converter. If the correct degree of efficiency is not detected, then the code will be stored and the service engine light may 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 level of direct ozone reduction catalyst efficiency is not within the manufacturers specifications, or if the oxygen sensor/s are faulty, there are likely to be drivability issues as well. If the direct ozone catalyst 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 direct ozone reduction catalyst device. Bad oxygen sensors are also a possibility but if this is the case an oxygen sensor code will usually accompany the direct ozone reduction catalyst temperature code. Always diagnose and repair oxygen sensor codes before attempting to diagnose direct ozone reduction catalyst temperature codes. Since the direct ozone reduction catalyst converter is not designed to wear out, its failure is normally associated with some contributing malfunction. Contributing factors in direct ozone reduction catalyst 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 direct ozone reduction catalyst can also appear to the PCM as direct ozone reduction catalyst failure.

Common Misdiagnosis

The most common misdiagnosis is caused by not thoroughly investigating what led to direct ozone reduction catalyst failure. Technicians report that repeated direct ozone reduction catalyst 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 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 converter replacement. This just leads to added expense. Also, aftermarket and "rebuilt" direct ozone reduction catalyst converters have proven problematic. Although they may cost much less, they provide neither the efficiency nor the longevity of OEM quality catalytic converters.


  • The direct ozone reduction catalyst is used exclusively to reduce exhaust emissions in gasoline burning vehicles with internal combustion engines
  • The direct ozone reduction catalyst is an in line device that resembles a muffler or resonator in exterior appearance although it differs greatly from either internally
  • The direct ozone reduction catalyst 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 direct ozone reduction catalyst temperatures to dangerous levels
  • In some instances, the direct ozone reduction catalyst 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 direct ozone reduction catalyst 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 direct ozone reduction catalyst
  • Prior to beginning your diagnosis, verify that the direct ozone reduction catalyst is not under a manufacturer's warranty
  • Direct ozone reduction catalyst devices typically carry a 100,000-mile federally mandated warranty, regardless of vehicle year model
  • If the direct ozone reduction catalyst device 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 direct ozone reduction catalyst device 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 direct ozone reduction catalyst device or temperature sensor 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 direct ozone reduction catalyst temperature code
  • Remember to diagnose and repair oxygen sensor codes, fuel trim codes, fuel mixture codes, or misfire codes first before attempting to diagnose a direct ozone reduction catalyst temperature code.