Troubleshooting my check engine light!

Hello all…

After 6 months of various projects my '89 XJ40 (all that I did myself) which included rebuilding the top end of the engine because of a blown head gasket, replacing catalytic converter, new Bilstein rear shocks and road springs, rebuilding the power steering pump and replacing the steering rack, pulling the gas tank to repair a pesky weep) etc, etc, the car is running and handling perfectly … except for a !#$% persistent check engine light that reads code 4 (O2 sensor circuit).

So far I’ve …

Replaced the O2 sensor with a new Bosch unit …
Just to be sure I also ran a bench test on it with a blowtorch … checked good. The O2 heater is getting 12v as it should.

Replaced the MAF air sensor with a used unit …
Both units checked good with a bench voltage check. Tried the second MAF just to be sure. Resulted in no difference in time it takes for CEL to come on.

Installed new front catalytic converter …
Didn’t really expect this to make a difference since the O2 sensor is installed upstream of the CAT. But since it had never replaced what the heck.

This has left me scratching my head and wondering where to go next with my troubleshooting.

Since the O2 sensor reports directly to the Engine Control Unit I guess my next step is to replace the ECU with a used unit to see if that makes a difference.

Since the engine runs beautifully I know I could just ignore the light and get on with my life, but … I JUST CAN’T ! (I’m cursed with that kind of personality :neutral_face:)

Any clever ways of troubleshooting to eliminate the ECU as the problem before I start replacing good parts ??

Now this is just a WAG …but …the O2 sensor gets it’s ground connection through the threads …maybe if you used anti-seize the sensor isn’t making a good ground - or maybe the o2 sensor is defective.
You could always run an “extra” ground wire to the sensor from a good ground on the car somewhere …

Just puttin’ it out there …

Groove - what criteria did you use to determine the oxy sensor is good? Voltage when fully warmed up / hot? Have you performed the same test at the connector to the ECU with the oxy sensor in situ and the engine fully warmed up / hot? Looking for a possible wiring issue here…

Larry …

Good thought. I’ve learned quite a lot about O2 sensors lately. You are absolutely correct that the 3 wire sensor in my car uses 2 wires for the heater (one of which is a ground) and the third wire is the signal wire with the body of the sensor being the signal ground. I’ll double check that the threads on the O2 sensor and catalytic converter header pipe are nice and clean.

Mike …

I hooked the positive lead of my volt meter to the signal wire and the negative lead to the sensor threads. I then heated up the sensor tip with my blow torch and the volts increased from 0 to about 1 volt and then moved back and fourth between that range. Everything i’ve read said this was correct.
I’m also getting 12v from the heater wires with the sensor hooked up and the ignition on

I wish I could pull a quick test to see if I have good continuity on the signal wire all the way to ECU without digging under the dash physically get to it. It looks like that may be my next step.

By way of troubleshooting to see if the O2 sensor was sending bad information to the ECU I installed an extender between the header and the sensor …

Screen Shot 2020-03-05 at 10.27.46 PM

… supposedly this helps to somewhat isolates the sensor from the exhaust so that even if you do have a lean or rich mixture the sensor won’t pick it up and trigger the CEL. Everything I read said this really works, but I still got the light at exactly the same time. This makes me think that the sensor itself isn’t the problem. Remember this is a brand new sensor that appears to test OK and the light is coming on after the same amount of time with the older sensor.

If both sensors, new and old, both triggered the same fault then by my reasoning the sensors are probably not the problem. Must be a communication issue so connections then I guess, eh? Is there a relay involved on your 89 car? if not I guess the only other thing is the harness or the plug/connector at the ECU.
You didn’t put any dielectric grease in any of the connectors, right?

…and the fuse boxes are OK too, right?

I had a failure-to-solve check engine light on my old pig of a thing (Range Rover) for years … I paid a mechanic to install two new oxygen sensors (dual exhausts) and it still didn’t cure the issue. Turns out that when the right side sensor was installed, it pushed one of the pins in the connector back and of course there was no connection so …check engine light!

Well my thought process so far is this …

What if everything is working as it should and the engine really is running too rich and triggering the O2 sensor to signal the CEL ?

A. I took my car to an inspection center. Of course they now check emissions via car’s computer
using an OBD2 scanner. They agreed to fire up their now obsolete rolling roadway and
placed the emissions sensors in the tailpipes. The results showed just a little rich but not
nearly enough to trigger the CEL.

B. If the O2 sensor was actually detecting a problem and tripping the CEL everything I’ve read
said that even if that were the case that using the sensor extender would remove the sensor
from the direct exhaust flow and “fool” it into not triggering the CEL.

C. The CEL always comes on when the car is first started from cold at about the exact same time,
roughly 4 minutes and sooner when the car is started from warm. This seems to be about the
time it takes the O2 sensor to warm up to app. 600 degrees (which is the temperature I believe
that the O2 sensor triggers the ECU to switch the system from open to closed loop) ?

I’m starting to believe that the ECU itself may be the culprit. Although I believe it’s suppose to go through a self test every time it’s energized I don’t know if it would display one of the 8 codes if it had an internal fault …

Screen Shot 2020-03-06 at 4.50.51 PM

Groove - because your bench test verified the O2 sensor is ‘generating’ the correct signal (voltage) my thought is the ECU is not RECEIVING that signal. As I previously stated I would suspect a wiring - or as Larry suggested - a connector issue. Checking the signal at the connector to the ECU would seem to me to be the next step. Bite the bullet and do it! :grin:

Mike and Larry …

A bad connection or broken signal wire does make a lot of sense.

Tomorrow I’m going to dig out the ECU and do a continuity check on the signal wire from the O2 sensor all the way back to ECU. Fortunately I have an excellent wiring diagram on my '89 model.

On the bright side if I don’t find anything wrong at least I’ll be able to read the part number on the ECU to order the correct used one.

Alright … here’s where I’m at

Opened up the passengers side (LHD car) kick panel and dug out the ECU. By using the wire color codes I was able to trace the signal wire from the O2 sensor all the way to the ECU. It’s a solid blue wire that’s a heavier gauge than all the others. Using a long jumper wire I was able to confirm that the wire has good continuity and therefore not broken. I’m also getting 12v through the two O2 sensor heater wires so no problem there.

Since I was able to determine the part number from my original ECU I ordered a warranteed used unit.
This is crazy because there are like a hundred different ECU part numbers for this car. Anyway I installed the new/used ECU and … no difference … argh ! The car starts right up and runs smooth as silk but after about 5 minutes the CEL code 4 comes on.

So I’ve now changed or tried …

  1. The O2 sensor (I bought yet another new one just to be triple sure)
  2. The MAF unit
  3. The ECU unit
  4. New CAT
  5. A new fuel pressure regulator
  6. A sensor extender to get the O2 sensor out of the exhaust flow

I’m sure there MUST be a way to troubleshoot this to narrow down the culprit but I’m scratching my head. But I’m not discouraged, I’ve cracked bigger nuts than this :sunglasses:

How about remove that extender thing?

Groove, Groove, Groove…do you remember Joe U’s lengthy no-start saga of last year? Methinks you could have continuity through the wire but the current could be compromised (although Joe’s problem was ultimately a dodgy relay rather than a wire). Have you checked the signal (voltage) at the ECU when the engine is warm and the CEL comes on?

About 6.

You don’t still have this extender installed, do you? Mayhaps I’m being dense, but it seems to me the O2 sensor must be in the exhaust flow to do its job, yes?

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No i removed the extender before my last round of tests.

OK, this is completely out in left field but recently when I was removing and replacing my downpipe where the O2 sensor is screwed in, I ended up having to replace the O2 sensor and since the only ones I had in stock were four wire O2 sensors versus the original three wire one, I ended up running the fourth wire from the wiring connecter on the O2 sensor harness to a intake manifold bolt where there are other EFI ground connections. The only difference between a three wire sensor and a four wire one is that the four wire ones do NOT rely on the ground passing through the threads of the sensor and into the exhaust. Instead they have a fourth wire that is hooked to a reliable ground, preferably the same one as the rest of the EFI management system so as to not cause a ground fault.

Now I don’t have a clue if a faulty ground passing through the exhaust is your problem but I am just providing this as an option. BTW, I feel your pain from this extended troubleshooting session for you CEL.

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Steven …

Good thought about the ground. Just to be on the safe side before I installed my new O2 sensor I used a small wire brush and cleaned the heck out of the threads in the exhaust pipe till they were nice and shiny. BUT , just to be surer I’ll solder a wire onto the body of the sensor and run it to a reliable ground.

Mike …

I can take a jumper wire and plug it into the O2 sensor’s signal output connector. I can then run it all the way to the ECU, bypassing the original wire completely. That means removing all the panels to get back to the ECU, but so be it if that completely eliminates the ECU (replacement behaving exactly like the original), sensor wiring, and new O2 sensor.

Strangely the sensor wire is a rather small gauge all the way out of the engine compartment but a larger gauge going into the ECU. Since I’ve owned the car for 28 years I’m fairly certain no one has modified the original wiring.

You could tapp into the signal at the ECU connection and watch it on a voltmeter as you are driving to see if anything strange happens at that 5 minute point.

Bingo! That’s the idea I was after:

OK then …

What I’ll do next is check the O2 signal at the sensor’s connector by pulling back the rubber cover and placing the positive lead of my multimeter on the connector and the negative lead to a good ground.
I’ll do this with sensor pugged in and the engine running and warm. From what I’ve researched the voltage reading should fluctuate rapidly between 1 and .1 volts. The richer the exhaust the average between the fluctuation should be closer to 1v and the leaner closer to .1v. A good old fashion osciliscope would be great for this… wish I had one.

Whatever this reading turns out to be it should read the same when I repeat the test at the ECU. At least I’ll definitely have eliminated the signal wire as the culprit.

If all this checks out it wold appear that the ECU and O2 sensor are working properly and telling me that something is actually causing the engine to run rich.

Groove, I would start the test with the engine cold and then you can observe the point when the o2 sensor comes into play.

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Hi Grooveman, In an earlier post you mentioned having the exhaust tested and it was slightly rich but not enough to trigger the CEL.
I have just dug out the Emisions Test Results from the MOT for my car in December.
The Lambda reading allowed is MAX LIMIT 1.030 MIN LIMIT 0.970.
It doesn’t say ppm or % vol as it does for the HC and the CO so I don’t know exactly what the Lambda numbers represent, but, I think you’ll agree it’s not much between pass and fail.
I’m certain Jaguar would have known the permissable limits when they programmed the ecu so anything outside those figures would naturally trigger the CEL. So maybe you are right and it is that 'slightly rich ’ condition that’s triggering the CEL all along.
My car passed with a reading of 1.023.
The full readout if you want to compare it with the readings for your car.
CO % Vol - 0.062 ( Max Limit 0.200 ) both at 1500rpm engine warm.
HC.ppm vol.- 46 ( Max Limit 200 )
I hope you get there soon, I’m sure you will.