Page 1 of 3
FORD MOTOR COMPANY REVISION DATE: JULY 13, 2011 PAGE 71 OF 246
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.
Bosch UEGO sensor interface:
IP – primary pumping current that flows through the sensing resistor
IA – current flow through trim resistor in parallel with sense resistor.
VM – Virtual ground, approximately 2.5 volts above PCM ground.
RE – Nernst cell voltage, 450mv from VM. Also carries current for pumped reference.
H+ – Heater voltage – to battery.
H- – Heater ground side – Duty cycle on/off to control sensor temperature.
Detection
Cavity
Reference Air
Diffusion
Passage
Heater
O
-
Exhaust
Gasses
O2,
HC,CO
NOx, H ...
Trim Resistor
(30 – 300 Ohm)
Pumping Cell
Nernst cell
(EGO)
Connector
IP
IA
IN
RE
H
-
H
+
Measurement
Resistor
(61.9 Ohm)
B
+
Heater Groun
Impedance
Measuremen
+ -
Pump
Current
Measurement
+
-
450
mV
ref. +2.5V
Virtual
Total
Pumping
Current
Measured
Pumping Current
Measured
Impedanc
e
PC
M
Senso
r
20ua reference pump
O
-
Bosch LSU 4.9
Page 2 of 3
FORD MOTOR COMPANY REVISION DATE: JULY 13, 2011 PAGE 72 OF 246
NTK UEGO sensor interface:
IP – primary pumping current that flows through the sensing resistor
COM – Virtual ground, approximately 3.6 volts above PCM ground.
VS – Nernst cell voltage, 450mv from COM. Also carries current for pumped reference.
RL - Voltage input from label resistor.
H+ – Heater voltage – to battery.
H- – Heater ground side – Duty cycle on/off to control sensor temperature.
Detection
Cavity
Reference Air
Diffusion
Passage
Heater
O
-
Exhaust
Gasses
O2,
HC,CO
NOx, H ...
Label Resistor
(3.5k – 1m Ohm)
Pumping
Cell
Nernst cell
(EGO)
Connector
IP
COM
VS
H
-
H
+
Measurement
Resistor
(300 Ohm)
B
+
Heater
Ground
Impedance
Measuremen
+ -
Pump
Current
Measurement
+
-
450
mV
ref. +3.6V
Virtual
Total
Pumping
Current
Measured
Pumping Current
PC
M
Sensor
reference pump
O
-
Multiplex
Voltage
NTK ZFAS-U2
RL- 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.