Front UEGO Monitor

Front UEGO Signal 

The UEGO sensor infers an air fuel ratio relative to the stoichiometric (chemically balanced) air fuel ratio by balancing the amount of oxygen pumped in or out of a measurement chamber. As the exhaust gasses get richer or leaner, the amount of oxygen that must be pumped in or out to maintain a stoichiometric air fuel ratio in the measurement chamber varies in proportion to the air fuel ratio. By measuring the current required to pump the oxygen in or out, the air fuel ratio (lambda) can be estimated. Note that the measured air fuel ratio is actually the output from the UEGO ASIC pumping current controller and not a signal that comes directly from the sensor.

Fig 1: Mode $06 Data Bosch UEGO Sensor Interface (Bosch LSU 4.9)

Courtesy of FORD MOTOR CO.

Fig 2: Mode $06 Data NTK UEGO Sensor Interface (NTK ZFAS-U2)

Courtesy of FORD MOTOR CO.

The primary component of a UEGO sensor is the diffusion passage that controls the flow of exhaust gasses into a detection cavity, a Nernst cell (essentially an EGO sensor inside the UEGO sensor) that measures the air fuel ratio in the detection cavity. A control circuitry in the ASIC chip (mounted in the PCM) controls the pumping current (IP) to keep the detection cavity near stoichiometry by holding the Nernst cell at 450 mV. This Nernst cell voltage (RE, VS) is 450mV from the virtual ground (VM, COM), which is approximately 2.5V (Bosch UEGO) or 3.6V (NTK UEGO) above the PCM ground. For the Nernst cell to generate a voltage when the detection cavity is rich, it needs an oxygen differential across the cell. In older UEGO (and HEGO) sensor designs, this was provided by a reference chamber that was connected to outside air through the wire harness that was subject to contamination and "Characteristic Shift Down (CSD)". The new UEGO sensor uses a pumped reference chamber, which is sealed from the outside to eliminate the potential for contamination. The necessary oxygen is supplied by supplying a 20 uA pumping current across the Nernst cell to pump small amounts of oxygen from the detection cavity to the reference chamber. The pumping cell pumps oxygen ions in and out of the detection cavity from and to the exhaust gasses in response to the changes in the Nernst cell voltage. The pumping current flows through the sense resistor and the voltage drop across the sense resistor is measured and amplified. Offset volts are sent out of the ASIC to one of the PCM's A/D inputs. The PCM measures the voltage supplied by the ASIC, determines the pumping current, and converts the pumping current to measured lambda. In general, the circuitry that measures the pumping current is used to estimate the air fuel ratio in the exhaust system.

The UEGO sensor also has a trim (IA) or label resistor (RL). The biggest source of part to part variability in the measured air fuel ratio is difference in the diffusion passage. This source of variation is simply the piece-to-piece differences from the manufacturing process. To compensate for this source of error, each sensor is tested at the factory and a trim or label resistor is installed in the connector. The value of this resistor is chosen to correlate with the measured difference between a particular sensor and a nominal sensor.

For NTK UEGO, the variation in the Ip signal value is corrected for by a compensation coefficient (CC), and then processed by the PCM. The value of CC (Ip rank) is determined by the value of RL. The PCM must command the ASIC to read the value of RL, so CC can be determined. After measuring the value of the label resistor, the PCM software will multiply the measured pumping current (Ip) by a compensation coefficient and determine a corrected pumping current that is used to calculate the measured exhaust air fuel ratio. During each power up, the PCM will briefly turn the UEGO heater power off, measure the output voltage from the voltage divider several times, average it, and estimate the resistance of the label resistor. The PCM will do this estimation multiple times, and if all samples are consistently within one resistor "rank", then the RL compensation coefficient determination is completed and the resistor "rank" compensation coefficient value will be stored in keep alive memory. On the other hand, if the several readings are not consistently within one rank for some amount of time, then the PCM A/D input is considered not reliable/RL erratic, and a trim circuit erratic malfunction (P164A, P164B) will be set. Conversely, if the estimated resistance is too high, then the software in the PCM will indicate RL circuit shorted to ground or open, and a trim circuit low malfunction (P2627, P2630) will be set. If the estimated resistance is too low, then the software will indicate RL circuit shorted to power, and a trim circuit high malfunction (P2628, P2631) will be set. Once a trim circuit malfunction is detected, then the compensation coefficient of the label resistor "rank" stored in KAM will be used.

For Bosch UEGO, the trim resistor is connected in parallel to the pumping current sense resistor and the pumping current flows through both. The trim resistor adjusts the measured pumping current back to the expected nominal value at any given air fuel ratio (correcting for the sensor to sensor variations in the diffusion passage). Small trim resistors are required for sensors that require more pumping current at any particular lambda. Conversely, for sensors with lower diffusion rates than average, less pumping current is required, so a higher than average impedance trim resistor is installed. When IA circuit is open, all of the pumping current flows through the measuring resistor which increases the measured voltage. Since the pumping current is amplified, the UEGO pumping current to lambda transfer function will reflect the error. The slope of the UEGO sensor transfer function changes, which results in the wrong output of the UEGO signal (the slope of the pumping current to lambda relationship can increase or decrease). For "stoichiometric" air/fuel control applications, an open IA circuit is not monitored since the lambda error is minimal in "stoichiometric" mode. A worst case (40 ohm resistor) open IA was tested on a 2008 MY 3.5L Taurus PZEV and showed no impact on tailpipe emissions.

The time spent at the limits of the short term fuel trim is monitored after vehicle startup when closed loop fuel has been requested, during closed loop fuel conditions, or when open loop fuel has been requested due to UEGO sensor fault. Excessive time with short term fuel trim at its limits (up to +/- 40%), or no rich / lean activity seen since startup indicates a "lack of switch" malfunction. Since "lack of switching" malfunctions can be caused by UEGO sensor malfunctions or by shifts in the fuel system, DTCs are stored that provide additional information for the "lack of switching" malfunction. Different DTCs indicate whether the sensor always indicates lean (P2195, P2197), or always indicates rich (P2196, P2198).

For Bosch UEGO applications, the time spent when the measured lambda is nearly 1.0 is also monitored after vehicle startup when closed loop fuel has been requested, during closed loop fuel conditions, or when open loop fuel has been requested due to UEGO sensor fault. Excessive time without measured lambda deviating from 1.0, in spite of attempts to force activity (via fuel control and reference current "defib") in the measured lambda, will indicate either a "lack of movement - open Pump Current circuit" malfunction or a "lack of movement - open Reference Ground circuit" malfunction. An open Pump Current circuit (IP) is differentiated from an open Reference Ground circuit (VM) by measuring the movements in the measured lambda during the reference current defib. Change in lambda movement below a minimum threshold indicates "lack of movement- open Pump Current circuit" malfunction, which results in P2237, P2240 DTCs (replaced P0134/P0154 DTCs). Conversely, change in lambda movement greater than the minimum threshold indicates an open VM, which results in P2251, P2254 DTCs (replaced P0130/P0150 DTCs).

Fig 3: needs caption

Courtesy of FORD MOTOR CO.

Since the Bosch CJ125 or the Conti-Siemens ATIC42 ASIC do not have the capability to specifically detect an open RE or VM, separate diagnostics were created to monitor these failures. An open RE or VM will typically cause the impedance of the Nernst cell to increase. An open RE will cause the UEGO voltage to be greater than or less than a malfunction threshold while an open VM will cause the UEGO voltage to be within a malfunction band. Note that this open VM detection will only enable if the UEGO is unable to control the heater voltage at the desired set point; otherwise, the "lack of movement- open Reference Ground circuit" diagnostic will enable.

UEGO equipped vehicles monitor the circuitry between the PCM and the UEGO sensor via the wire diagnostics capability included on the UEGO ASIC chip. The wire diagnostics will detect wires (IP, IA, VM/COM, RE/VS) shorted to battery, or ground, and in most cases will detect open circuits (IP, VM/COM, RE/VS). The diagnostic bits are transmitted to the PCM via SPI (serial peripheral interface). The SPI communication is validated continuously, and if a SPI communication failure is detected, fault code(s) P064D and/or P064E will be set. The ASIC is also capable of detecting internal circuitry failure; in which case, an ASIC failure DTC (P1646, P1647) along with the SPI communication failure DTC (P064D, P064E) will be set.

• Beginning 2011 MY, the general UEGO circuit diagnostic DTCs P0130/P0150, are now replaced by more specific DTCs.
• A shorted to ground circuit (Bosch UEGO - IP, IA, RE, VM; NTK UEGO - IP, VS, COM) will set P0131/P0151 DTCs.
• A shorted to battery circuit (Bosch UEGO - IP, IA, RE, VM; NTK UEGO - IP, VS, COM) will set P0132/P0152 DTCs.
• An open Pump Current circuit (IP) will set P2237/P2240 DTCs.
• An open Reference Ground circuit (VM/COM) will set P2251/P2254 DTCs.
• An open Reference Voltage circuit (RE/VS) will set P2243/P2247 DTCs.

For "Non-Stoichiometric Closed Loop (NSCL)" air/fuel control applications, a continuous open IA diagnostics (Air Rationality Test) is required since the lambda error is more significant in this mode. The air rationality test will always monitor the UEGO sensor voltage or pumping current reading during Decel Fuel Shut Off (DFSO) event. The monitor compares the UEGO sensor voltage or pumping current reading in air against the expected value for pure air. If the UEGO sensor voltage or pumping current during DFSO exceeds the maximum UEGO voltage/pumping current in air threshold, then the fault timer increments. If the fault timer exceeds the fault time threshold, then open IA DTC P2626 and/or P2629 will set. Since transient sources of fuel in the exhaust after injector cut can contribute to the UEGO sensor voltage/pumping current to read lower (rich), the air rationality monitor will not call a pass until the transient sources of fuel have been exhausted and pure air entry conditions during DFSO are met (i.e. all injectors must be off, purge must be off, no fuel must be leaking around the PCV valve, and a few transport delays must have passed to allow the last fuel transients to be exhausted leaving nothing for the sensor to see, but air). Note: Beginning 2011 MY and beyond, this diagnostics will monitor the UEGO pumping current against the expected value for pure air instead of the UEGO voltage so the monitor can be ASIC chip independent.

Fig 4: Mode $06 Data For "Non-Stoichiometric Closed Loop (NSCL)

Courtesy of FORD MOTOR CO.

Front UEGO Slow/Delayed Response Monitor (2010 MY+) 

The front UEGO monitor also detects malfunctions on the UEGO sensor such as reduced response or delayed response that would cause vehicle emissions to exceed 1.5x the standard (2.5x the standard for PZEV). The response rate is evaluated by entering a special 0.5 Hz square wave, fuel control routine. This routine drives the air/fuel ratio around stoichiometry at a calibratable frequency and magnitude, producing predictable oxygen sensor signal amplitude.

A UEGO slow or delayed sensor will show an increased response time which is compared to a no-fault polygon. Combinations of the rich to lean and lean to rich response times that fall outside the polygon indicate a sensor malfunction (P0133 Bank 1, P0153 Bank 2).

Fig 5: Mode $06 Data For Typical UEGO Response Rate Malfunction Thresholds

Courtesy of FORD MOTOR CO.

UEGO Heaters 

The UEGO heater is controlled as a function of the measured impedance to keep the sensor at a near constant temperature (Bosch: 780 deg C, NTK: 830 deg C). The impedance of the Nernst cell decreases as the sensor temperature increases. This impedance is measured by periodically applying a small current across the Nernst cell and measuring the change in the voltage. The output voltage is then sent to an A/D input on the PCM. After a cold start, the UEGO heater ramps up to the maximum duty cycle to heat the sensor. After a few seconds, the measured impedance will start to decrease and when the target value is crossed, the heater goes into closed loop heater control to maintain the sensor at a near constant temperature.

The "UEGO Heater Temperature Control Monitor" tracks the time at the maximum duty cycle during the open loop sensor warm up phase. If the measured impedance does not come down to the target value to allow the system to transition from open loop heater control to closed loop heater control within a specified time, then a P0030/P0050 fault code is set. This monitor also sets a malfunction when the closed loop heater controller reaches a maximum or minimum duty cycle for a period of time indicating that the controller is no longer able to maintain the target temperature. However, if the exhaust temperature is high enough that the sensor will be above the target temperature even with no heat, then this monitor is disabled.

The UEGO heaters are also monitored for proper voltage and current. A UEGO heater voltage fault (open, shorted to ground, or shorted to battery) is determined by turning the heater on and off and looking for corresponding voltage change in the heater output driver circuit in the PCM.

A separate current-monitoring circuit monitors heater current once per driving cycle. This monitor normally runs in closed loop heater control after all the exhaust gas sensor functional tests are completed (2010 MY and earlier), however, it can also run intrusively. When the UEGO sensor indicates cold, but the heater is inferred to have been adequately warm, the current monitor is forced to run intrusively prior to the completion of the heater temperature control monitor. If the measured heater current falls below or above a calibratable threshold, the heater is assumed to be degraded or malfunctioning.

Beginning 2012MY, some PCMs do not have a separate current-monitoring circuit. For PCMs that do not have the current-monitoring circuit, a degraded or malfunctioning UEGO heater is detected by the "UEGO Heater Temperature Control Monitor".

Fig 6: Mode $06 Data For Typical UEGO Heater Check Malfunction Thresholds

Courtesy of FORD MOTOR CO.

Fig 7: Mode $06 Data For Front UEGO Monitor

Courtesy of FORD MOTOR CO.