Jeep 4WD & Ram Truck How-to:
Always Use Genuine Mopar Oxygen Sensors!
Four-wheeling in the
Sweetwater Range or Nevada's remote high desert country, the Jeep Cherokee must perform
reliably. Genuine Mopar parts can make the difference...
Tuning and troubleshooting a Jeep engine means more than just parts
replacing. Mis-diagnosis or use of wrong parts can lead to wasted time and unreliable
While modern Jeep vehicles have onboard diagnostics (OBDII-up), these
systems cannot pinpoint every trouble. Some troubles require use of a precision scan tool, seldom
available in the backcountry. Some oxygen sensor troubles do throw a code (DTC) on the dashboard MIL or
"Engine Check" light. Sometimes, however, a defective or improperly calibrated oxygen sensor can create a
major drivability problem without ever sending a code to the dashboard MIL.
One difficult scenario is a defective or incorrect
replacement part that will not throw a trouble code (DTC). We typically assume that a new part moves
that item out of the trouble category. While this is true for genuine Mopar replacement parts, there is
often less certainty with aftermarket, generic parts.
A Glaring Example
A case scenario is the magazine's 1999 Jeep XJ Cherokee 4.0L. When the
Cherokee's original upstream #1 oxygen sensor seemed due for a routine replacement (at 135,000 miles), I
decided to replace the device.
sensor is the OEM for the 4.0L engine. This device still provided service at 135,000 miles. Inside is the new,
aftermarket sensor, the second new aftermarket sensor (from the same brand-X manufacturer) to create a dead
throttle after an engine restart—either hot or cold.
I have always valued and recommended Mopar replacement parts for our
Chrysler-built vehicles—and for good reason. Servicing
the Cherokee on a weekend, when the Jeep dealership's parts department was closed, I ignored my
better judgment and purchased an aftermarket oxygen sensor from the local automotive chain
Shortly after the new sensor's installation,
the 4.0L engine developed a
throttle flat spot. From just off-idle to approximately 1/4 throttle opening, the problem occurred
on an engine restart, around the temperature where the fuel-and-spark management shifts from
"open loop" (cold/warm-up phase) to "closed loop" (warmed engine). The idle was smooth, without
The problem was severe.
Unless you pressed the throttle past this position, the engine would lean backfire and act
fuel-starved. In traffic, this created a hazard. Once pushed past the throttle flat spot, the
engine smoothed out and performed properly—until the next restart and warm-up cycle. Then the dead throttle
symptom would repeat itself.
This is the
OEM Mopar sensor in place. Bench testing revealed a still-functional device at 135,000 miles! The replacement
brand-X device faulted shortly after installation. A second brand-X device did the
I have never been a "parts replacer" kind of mechanic. Thorough
troubleshooting is always important, and in this case, I began systematically checking over the engine
devices and sensors, without suspecting the new oxygen sensor as a trouble source.
Given the warm-up issue and the throttle involvement, I naturally
focused on the throttle position sensor (TPS) and two temperature sensors (coolant and manifold intake air).
Since the engine ran fine once past the initial flat spot cycle, I ruled out the MAP and MAF
Considering the 135,000 miles on the vehicle, these were logical places
to troubleshoot. Each of these devices can be bench tested. The coolant and air temp sensors have
parameter tests, performed with an ohmmeter. The TPS is a variable voltage test, performed at
different throttle openings.
Each of these devices tested moderately out of range (based upon
the ohms or volts indicated in the factory Jeep service manual or data found at the Mopar TechAuthority II subscriber website). Given
the symptoms and a fresh oxygen sensor, I replaced each temperature sensor and the throttle position sensor.
This did not solve the problem.
Since the problem with the first aftermarket sensor did not occur with a warmed engine, I ruled out the MAP,
MAF and other devices as possible trouble sources.
Note: This guesswork would not have been necessary with the use of
a DRBIII scan tool. In many instances, it proves cost effective to sublet troubleshooting
to your local Chrysler dealership. They have diagnostic tools like the DRBIII, StarScan and, most
These are the
OEM Mopar pieces. Note the plastic intermediate sleeve and precision, four-prong plug. Aftermarket parts may
not be this precise and often lack the fit of genuine Mopar replacement
After a telephone discussion with a friend and Five-Star dealership Jeep
technician, I opted for a continuity and ohms resistance test of the recently installed oxygen
sensor. My friend had run into this problem before. He took a misbehaving (new at the time)
1999 TJ Wrangler for a test drive with the DRBIII in "pilot mode". He discovered a subtle
oxygen-sensor "blip" as the engine tipped over from open loop to closed loop...The problem went away by replacing
just the oxygen sensor—with a genuine Mopar warranty part.
The ohms and voltage tests revealed a discrepancy. When I returned
to the parts store, the manager studied the sensor and determined that the store had sold me the wrong item.
He asked me to install another sensor, with a different part number. We would see if that solved the
aftermarket oxygen sensor also failed. Parts replacing, without clear test results, is no way to
troubleshoot or eliminate problems and symptoms.
Installing the new aftermarket part, which incidentally fit a
wide variety of engine applications, the symptom persisted—only this time, the dead throttle occurred whether
the engine was cold, warming or fully warmed.
For proper fit, correct calibration and
reliability, use a genuine Mopar replacement sensor. Look for the "Mopar™" logo
on these parts.
The OE oxygen
sensor was an NTK unit from Japan, genuine Mopar part #56041212AE for this 1999 XJ Cherokee 4.0L
application. Despite the passage of a dozen model years, the new Mopar replacement part looks just like
the original part. Connections identical, not generic, the device fits just like the original upstream
Startup, warmup, driving in all modes, the engine now performs
optimally. Although there are other items that can cause the symptoms described, in this situation, the problem was
the oxygen sensor—specifically, an aftermarket, brand-X sensor!
Tightening a new Mopar oxygen sensor in place is time well spent. This device will
perform as the engineers at Jeep and Mopar intended.
is critical during sensor installation. This is a genuine Mopar sensor attaching to the original lead with four
pins. The upstream oxygen sensor is a heated type with four color-coded wires.
A DRBIII or equivalent scan tool can help pin
down a faulty oxygen sensor during a real-world set of tests. Without these troubleshooting tools, there
are bench tests for faulty devices. When you detect a problem, use OEM Mopar replacement parts that fit
properly and offer proper calibration. These are signals that the EFI/MPI system can
Note: When troubleshooting points to a
specific device, assure your project's success by using genuine Mopar replacement
An advantage with the Mopar part is direct fit to the factory engine bracket. The oxygen
sensor will align with the engine and chassis as intended.
Here, the new
Mopar oxygen sensor plug fits precisely to the OEM engine bracket. The wire will stay out of harm's
these parts in Jeep 4WD service, both on- and off-highway. Take care to route wire aware from the manifold's heat.
As an extra precaution, secure the wire heat shield with a tie-strap. This will prevent the sleeve from
sliding and exposing wires to extreme exhaust manifold heat.