NOx Sensor Circ Range/Perf Bank1


The PCM has detected a NOx (nitric oxide and nitrogen dioxide gases) sensor circuit voltage reading that is not within the normal operating threshold for a specific engine bank. The PCM utilizes exhaust composition and temperature data from the oxygen sensors to calculate typical NOx sensor circuit input data. If this level is not detected, then the code will be stored and the service engine light will be illuminated. Bank 1 refers to the engine bank which contains the number one cylinder. See the manufacturer’s service manual 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 the desired NOx sensor readings for each respective catalytic 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 (including NOx) in gasoline and diesel burning vehicles
  • 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 an infrared temperature gun with a laser pointer 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
  • Start the engine and allow it to reach normal operating temperature
  • Raise the vehicle on a suitable lift and secure it
  • Point 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 and NOx sensor operation on the affected engine bank
  • The “heater” portion of the NOx sensor is an electrical circuit that is dedicated exclusively to warming up the sensor/s
  • Most NOx sensors in OBD-II equipped vehicles are of the heated variety
  • Each heated oxygen sensor has its own integrated heater
  • By heating the NOx sensor using electrical voltage, the PCM is permitting it to become operational sooner than if it was heated using only exhaust gases
  • This decreases emissions and the time needed for the PCM to enter closed loop mode. Most heated NOx sensors use a battery voltage (approximately 12.6 to 13.8-volts)
  • On some models the voltage is delivered via a fused circuit
  • The heated NOx sensor circuit is completed with a ground supplied by the PCM (with the KOEO or KOER)
  • Other manufacturers use a computer controlled supply of battery voltage and a continuous ground (no fuse in needed in this design)
  • You may choose to test voltage to the individual sensor heater circuits (power and ground) using a voltage drop method
  • Test voltages should be near that of the fully charged battery
  • If the PCM detects a fault in heater system circuit voltage (or resistance) a code will be set and a service engine soon lamp may be illuminated. If no obvious problem is found, then use a scanner to view data from the scanner in question
  • Narrowing the scanner’s data stream to include only this sensor will increase the accuracy and response time of the data stream
  • Using a digital volt/ohmmeter to obtain live data is also an acceptable method for monitoring sensor function and will provide you with more accurate readings
  • Upstream (pre catalytic converter) heated NOx sensor readings should constantly fluctuate between approximately 100-millivolts and 900-millivolts, after the engine reaches normal operating temperature and the PCM enters closed loop operation
  • Downstream NOx sensors should find a point near the center of a lean and rich condition and remain within 100 to 200 millivolts of that point until a significant engine RPM change is actuated
  • If your live data testing (or the scanner data display) reveals a similar reading, then unplug the NOx sensor connector in question and inspect the pins for signs of damage or corrosion
  • If the connector is faulty, repair or replace it as required and clear the codes
  • Test drive the vehicle to make sure that the repair was successful. If the connector pins appear to be in good shape, then prepare to perform resistance and continuity testing on the NOx sensor by disconnecting the electrical connector
  • If live data suggests that the NOx sensor is not performing properly, disconnect the connector and test the sensor itself
  • If the battery voltage feed circuit for the sensor heater indicates “no resistance”, suspect that it is defective
  • Test the NOx sensor as suggested by the manufacturer and compare your findings with manufacturer’s specifications
  • Replace the sensor as needed and clear the codes afterward
  • Test drive the vehicle afterwards to ensure that a successful repair was performed. If the NOx sensor conforms to the manufacturer’s specifications, prepare to test system circuits for resistance and continuity by disconnecting the electrical connector/s from all related control modules (especially the PCM)
  • Consult the manufacturer’s service manual and perform continuity and resistance testing on all system circuits
  • Compare your actual findings with manufacturer’s specifications and repair or replace circuits, components, or connectors as required
  • Once the repairs are made, clear the codes and test drive the vehicle to ensure that the repair was successful. If NOx sensor and system circuits are comparable to the manufacturer’s specs, suspect a defective PCM (or other related controller)
  • Controller failure is rare and replacement of such will require reprogramming. Remember to diagnose and repair oxygen sensor codes, fuel trim codes, fuel mixture codes, or misfire codes first before attempting to diagnose a NOx sensor circuit code