The Powertrain control module (PCM) will provide a .45 volt reference voltage
to the Oxygen sensor. When the O2 sensor reaches operating temperature, it will
generate a voltage that will vary depending on the oxygen content of the exhaust.
Lean exhaust generates a low voltage (less than .45V) and rich exhaust generates
a high voltage (greater than .45V). O2 sensors on a specific bank marked as
"sensor 2" (as this one is) are used to monitor emissions. A Three-Way
Catalyst (TWC) system (catalytic converter) is used to control tailpipe emissions.
The PCM uses the signal received from Oxygen sensor 2 (#2 indicates aft of catalytic
converter, #1 indicates pre-converter) to read efficiency of TWC. Normally this
sensor will switch between high and low voltage at a noticeably slower rate
than the front sensor. This is normal. If the signal received from rear (#2)
O2 sensor indicates that the voltage has "stuck" between .425V to
.474 V, the PCM determines this sensor is inactive and this code will set.
Potential Symptoms
Your check engine light(CEL), or malfuction indicator lamp (MIL) will be illuminated.
There will not likely be any noticeable drivability problems other than the
MIL. The reason is this: The rear or post catalytic converter Oxygen sensor
does not affect fuel deliver(this is an exception on Chryslers). It only MONITORS
the efficiency of the catalytic converter. For this reason, you will likely
not notice any engine trouble.
Causes
The causes for a P0140 code are fairly few. They could be any of the following:
- Shorted heater circuit in O2 sensor. (Usually requires replacement of heater
circuit fuse in fuse block also)
- Shorted signal circuit in O2 sensor
- Melting of harness connector or wiring due to contact with exhaust system
- Water intrusion in harness connector or PCM connector
- Bad PCM
Possible Solutions
This is a fairly specific problem and shouldn't be too difficult to diagnose.
First, start engine and warm up. Using a scan tool, watch the Bank 1, sensor
2, o2 sensor voltage. Normally the voltage should switch slowly above and below
.45 volts. If it does, the problem is likely intermittent. You'll have to wait
for the problem to surface before you can accurately diagnose.
However, if it doesn't switch, or is stuck then perform the following: 2. Shut
off vehicle. Visually check the Bank1,2 harness connector for melting or chafing
of the harness or the connector. Repair or replace as needed 3. Turn ignition
on, but engine off. Disconnect the O2 sensor connector and check for 12Volts
at the Heater Circuit supply and for proper ground on the heater circuit ground
circuit. a. If 12V heater supply is missing, check the proper fuses for an open
in the circuit. If heater circuit fuse is blown, then suspect a bad heater in
the o2 sensor causing a blown heater circuit fuse. Replace sensor and fuse and
recheck. b. If ground is missing, trace the circuit and clean or repair ground
circuit. 4. Next, with connector still unplugged, check for 5 Volts on the reference
circuit. If this is missing, check for 5 Volts at the PCM connector. If 5 Volts
is present at the PCM connector but not at the o2 sensor harness connector,
then there is an open or short in the reference wire supply between the PCM
and the o2 sensor connector. However, if there is no 5 Volts present at the
PCM connector, the PCM is likely at fault due to internal short. Replace PCM.
** (NOTE: on Chrysler models, a common problem is the 5Volt reference circuit
can be shorted out by any sensor on the car that uses a 5 Volt reference. Simply
unplug each sensor one at a time until the 5 Volts reappears. The last sensor
you unplugged is the shorted sensor. Replacing it should fix the 5 Volt reference
short.) 5. If all the voltages and grounds are present, then replace the Bank
1,2 O2 sensor and re-test.
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