O2 Fault Codes

Hello, in the last month my Jeep Liberty KJ 3.7 2002 has been running very bad and the engine light has been blinking from time to time. Just cleared the codes and these came up straight away. 1 of them read a to high voltage. Are these a common problem for the jeep? And will these make the engine misfire? Any help is appreciated!

Yes I think if your O2 sensor is sending the wrong reading to the ECM, you might not have the right ratio of gas:air and it can misfire and run badly.

On the other hand, if you had a pre-existing misfire and a bunch of unburned gas was going down the exhaust, it can destroy O2 sensors and catalytic converters. Chicken and egg problem.

How long has it been doing this, and do you smell gas when the engine is misfiring? If so, I wouldn’t run it very much or you could wipe out your cat. I would try replacing the O2 sensor and go from there.

Agree w lfhoward. 4 o2 sensors on a 2002, correct? (I have a 2003 and I know I have 4, never hurts to confirm) Upstream/downstream, driver side passenger. Cyl #2 and #6 are misfiring so the whole passenger side exhaust is getting gunked up.

Before you replace the sensor, 2 questions:
1. Are you getting any exhaust smoke at all, even a wisp?
2. When was the last time you replaced your plugs?

If you are getting smoke and you have >200K on the odo, you may be getting ready to blow a ring and need to have a shop really check it out. No smoke, read on and DIY.

Logically speaking dirty misfires mess up cat sensors, egg before chicken.

Misfires will cumulatively mess up O2 sensors, and if the root cause is incomplete combustion due to bad plugs... a replaced O2 it will just get filthy again... and you'll have to buy another O2 sensor again. NGK v-power copper plugs, gap at 0.40 (regardless of what the auto shop catalog or mfr posts). Check the problem plugs #2 and #6 1st and if their gap is way off and/or appearance is bad, then *replace all 6 plugs good or bad with a matching .040 gap* and do a hard reset on the PCM by disconnecting neg battery cable 1st, then pos cable, hook the 2 loose battery cable connectors together and let them sit for about 10 minutes before reconnecting in reverse order to the battery. That'll wipe all codes by completely draining the static memory of the PCM.

If misfire codes stay gone, drive it for a while and maybe the O2 sensor deposits will cook off, the O2 code disappears, and you save the replacement cost. Probably not, and the best you can hope for is the cost of just 2 new sensors... but the cat is probably trashed too.

If only the O2 code persists, then the up and down sensors are probably permanently damaged. Replace them with with NGK parts, make sure you identify the correct part numbers. All 4, up/down and pass/driver, are all unique part numbers. Double check them on the NGK web site.

I would replace the upstream sensor(between engine and catalytic) on the passenger side as the heater circuit code is a definite sign of a bad O2 sensor- they have slow reaction times when they are cold. The wiring for the downstream sensor on passenger side needs to be looked at as the wires might have insulation missing causing that shorted to voltage fault.
Any V6 or V8 will have 4 O2 sensors even ones with only one catalytic converter, but with one catalytic there will be no Bank 2 Sensor 2

O2 sensor failure is common, not just in Jeeps.
I'm always grateful for sensors that work hard in harsh environments.

If you suspect the air-fuel ratio adjustment system including the O2 sensor,
Advanced scanners can be used to read air/fuel ratio corrections to determine how well your system is doing.

Fix the misfire codes first. The faulty heater circuit isn't going to really contribute to your misfire problems, and the downstream sensors are primarily just used for determining if the cats are doing their job.

Isn't the downstream sensor also included in the air-fuel ratio adjustment loop?

Below is an excerpt from the service manual.

>>
Two upstream sensors are used (1/1 and 2/1).
The 1/1 sensor is the first sensor to receive exhaust gases from the #1 cylinder.
They provide an input voltage to the PCM.
The input tells the PCM the oxygen content of the exhaust gas.
The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensors.
The PCM will change the air/fuel ratio until the upstream sensors input a voltage that the PCM has determined will make the downstream sensors output (oxygen content) correct.
The upstream oxygen sensors also provide an input to determine mini-catalyst efficiency.
Main catalytic convertor efficiency is not calculated with this package.

I'm reliably informed they're all the same, except the plugs & the length of the wires. Certainly mine seems quite happy with 2 el cheapo universal ones &, I think, now both of those off my '96 XJ.

ikuo78 said:

Isn't the downstream sensor also included in the air-fuel ratio adjustment loop?

Below is an excerpt from the service manual.

>>
Two upstream sensors are used (1/1 and 2/1).
The 1/1 sensor is the first sensor to receive exhaust gases from the #1 cylinder.
They provide an input voltage to the PCM.
The input tells the PCM the oxygen content of the exhaust gas.
The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensors.
The PCM will change the air/fuel ratio until the upstream sensors input a voltage that the PCM has determined will make the downstream sensors output (oxygen content) correct.
The upstream oxygen sensors also provide an input to determine mini-catalyst efficiency.
Main catalytic convertor efficiency is not calculated with this package.

Click to expand...

There isn't a reason to look at the downstream sensor for adjustments to air/fuel as the catalytic is changing the results

The service manual continues.
This appears to be an uncommon control since both sensors are upstream of the main catalyst.
>>
Downstream Sensors - 3.7L Engine: Two downstream
sensors are used (1/2 and 2/2).
The downstream sensors are used to determine the correct airfuel ratio.
As the oxygen content changes at the downstream sensor, the PCM calculates how much air-fuel ratio change is required.
The PCM then looks at the upstream oxygen sensor voltage, and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content).
The downstream oxygen sensors also provide an input to determine mini-catalyst efficiency.
Main catalytic convertor efficiency is not calculated with this package.
Engines equipped with either a downstream sensor(s), or a post-catalytic sensor, will monitor catalytic convertor efficiency.
If efficiency is below emission standards, the Malfunction Indicator Lamp (MIL) will be illuminated and a Diagnostic Trouble Code (DTC) will be set.
Refer to Monitored Systems in Emission Control Systems for additional information.

The upstream and downstream sensors should all constantly bounce between a reading of around 0.1 and 0.85. If one of those sensors is stuck/not-bouncing that is the likely culprit.... but it also may be a bad cat on that side.

If all sensors are bouncning between 0.1 and 0.85, then it is likely not an o2 sensor because your reader is reading what the PCM sees, and it expects the bounce in that range.

The other thing to pay attention to is the fuel mix. If you have a decent reader, it should have the ability to show you live data... and if you can see live data look at the long and short fuel trim numbers. After the KJ PCM has dropped into closed loop state, Short Term FT and Long Term FT numbers should bounce around 0.0 (back and forth between plus or minus something). If the PCM is showing that fuel trim is staying consistently in a negative (rich) or positive (lean) condition, then you have faulty combustion somewhere... and you should see a P030x code meaning "misfire". If you see this behavior, check plugs for wear and gap... if not that then the root problem is further away from the cylinder and may be..
1. A bad ignition coil which is DIY and therefore cheaper than replacing injectors so try the coil on the bad cyl 1st...
2. Wiring. An exposed wire that is grounding will mess up the PCM. Also, you must have the connector housings for all connection able to fully lock because if anything is loose it will mess with the PCM feedback loop from wherever the loose connection exists, replacement connector housings manufactured by Standard and available from AZ/Napa can be DIY spliced cheaply...
3. A fuel injector, which is the most expensive fix and probably the culprit if you are ***still*** seeing a lean skew condition on the LTFT (clogged) reading, after you have checked and replaced bad spark plugs with (pure copper) plugs that are correctly gapped at .040

Again, if you eliminate everything except the o2 sensors... do not screw around with cheapo/universal parts. Mopar explicitly identifies 4 different part numbers, 1 for each of the 4 sensor positions in the exhaust system, for a reason.

Spend the money for the 4 specifically matched NGK (or spend more $$ and buy Mopar if they are still available, but NGK works perfectly) sensors. Or, risk chasing your tail/wasting time/wasting more $

O2 sensors are generic; Mopar, like everyone else, buys them by weight from whoever offers them cheapest. The only difference between the 4 is the length of the wires. As I've said before my KJ is perfectly happy (i.e it keeps the CEL out) with 2 el cheapo universal ones & the 2 salvaged from my '96 XJ.

You really want to see higher voltage on the O2's that are downstream of the catalytic converters as this shows that the excess fuel is being burned off and there is less oxygen. They shouldn't show over 0.1V as that would mean they are shorted to voltage somewhere.