1989 XJ40 Check Engine Light

Gentleman …

Ever since my recent head gasket replacement I’ve been getting a Check Engine Light on the VCM after about 5 -10 minutes of driving. The code comes up with a 4, which is “Oxygen sensor full rich”.

I replaced the O2 sensor with no change. Next I removed and thoroughly cleaned the Mass Air Flow sensor, with no change.

Any suggestions on where to go from here ?

Temperature sensor duff? ECU thinks its cold and running on choke?

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After a long ponder came up with the same as Robin …temp sensor. …either that, or you have an air leak …intake manifold gasket can often seep a small amount of unmetered air, happened to me, periodically got FF23 (RICH) before I got it totally sealed up.

Probably your temp sensor tho…:slightly_smiling_face:

The coolant temperature sensor was my first thought also … but ,.

… that would show up as a code #3 “Coolant temperature sensor (not in operating range)” not a code #4. But just to be on the safe side I replaced the coolant sensor with a new one with no change. I have had a #3 code in the past and changing the coolant sensor did solve the problem.

Regrettably there are only 8 codes available on my '89 model so really scant information. Starting on the '90 model the number of codes available went up to 24, still nothing compared to modern cars but
getting better.

So It’s either a false signal triggering the CEL or the car really is running very rich.

Usually the code pops up after about 5 minutes of driving, but after I cleaned the MAF I drove around for about 20 minutes before it reared it’s ugly face. Don’t know if this could possibly have any significance ?

Hi Grooveman. I have no manuals at the momen to check if this applies to xj40s, but I know lots of cars have an oxygen sensor heater, and a relay that operates it, if so maybe you could check that out.

I know you have serviced your injectors when doing the head job, but is there any chance that one of them is dripping rather than spraying and giving too much fuel ?

You could check the colour of your spark plugs to see if one or more are darker/ sootier than normal.
Do you have a local garage who could do an emissions test ?

When the book says " Oxygen Sensor Full Rich " I would assume that ‘Rich’ referred to the air/fuel ratio, is that correct ? or does it mean ‘Oxygen Rich’ and therefore the mixture is actually too weak ?

Have you back probed the o2 sensor to see what the actual reading is when the fault is thrown? That might indicate if there really is a problem.

Boy Grooveman you better hope it is not ‘Oxygen Rich’ i.e. lean. Can you say ‘burned valves’? I would say Casso is spot on, check the spark plugs and exhaust pipes for black soot. Black soot = too (fuel) rich. But I bet you already knew that.

Dennis, when the CEL comes on, do you feel any shaking from the engine? My car came with a insulation sleeve on the O2 sensor right where the wires enter the sensor. When I replaced it, the new one didn’t come with any sleeve and my car threw a code44 (rich or lean condition in the oxygen sensor circuit). I don’t know if the earlier cars require this sleeve or not but this was the reason the later cars came with it.

Gentleman thanks for the input …

I assume that the code “Oxygen Sensor - system indicates full rich” means just that, that the sensor is detecting a too (full) rich air mixture passing by it.

The sensor is heated. Is there a relay and how would I check that? The sensor does have a insulation sleeve.

*“Have you back probed the o2 sensor to see what the actual reading is when the fault is thrown?” Robin can you give me a little more information on doing that please.

I do have a very slight stumble when idling.

Could one of my injectors be the problem, that is a possibility. I have a new set of injectors that I can pop in and see what happens.

So where to go from here? …

I think I’ll start by pulling all the spark plugs and checking them. While I’m at it I’ll pull a compression check. The machinist who rebuilt my head took great pains in setting the valve clearances and so I just
installed it and moved on. Perhaps I needed to recheck the clearances after the head was bolted down?

I bought a new front CAT thinking that if the old one is partially blocked that “could” be causing this? The trouble is I can’t get to one of the bolts to remove the old one and install the new one. I took the car to a muffler shop and they couldn’t figure it out either. It almost looks like I’ll have to remove the two exhaust headers and pull the CAT out still attached to them … ouch !


The line from the o2 runs from the sensor across the top of the fire wall from memory, you should be able to get a probe in the connector (again from memory it’s white) ground one of the probes and with the engine running the volts should hover around .5v +/- .3v when the engine is up to temp.

Now here’s something interesting …

Still trying to track down my CEL. So I decided to disconnect the heater electrical connector to the O2 sensor and leave the sensor wire connected. Drove the car for 30 minutes in 90 degree temperature so the exhaust was nice and toasty … and no Check Engine Light !

As much as I could research heated O2 sensors on the internet it appears that all the heater does is warm up the sensor to get the engine computer into the closed loop (where it actually reads the info from the sensor and not a base line stored in the computer) quicker. Once the exhaust heats up the O2 sensor enough the heater doesn’t really do anything. So will a heated O2 sensor still work with the heater disconnected … everything I read said they do … Hmmm

That’s my understanding as well, I’m not sure why, with the o2 heater disconnected that you are not getting the MIL but hey take the win if thats what it is :slight_smile:

Robin …

Since I’m getting the Check Engine Light with both the old O2 sensor and the new replacement I think that kind of rules out the sensor as being the culprit. But how could the heater wiring upstream of the sensor be causing a CEL ?

TBH Groove no idea :frowning:

WAG - I had an ancient gas guzzlin’ Range Rover that consistently flagged CEL due to one of the 02 sensor plugs having a pin pushed back in the connector …maybe check and replug all the connectors in the circuit, fusebox and ECU too and see what happens.

Finally found the Rangie fault a week before I sold it. Gas station owner was quite upset it was gone as it helped him pay for his Alaska cruises.

Not long ago one of our wonderful forum members posted a link to an in-depth wiring diagram of the entire '89 model. And when I say in-depth I mean 67 pages ! It also includes every wiring connector along with it’s location and a color identification of each pin. I can’t recall who it was but thank you and if you read this please take a bow. After downloading it I labeled the different pages and put it in a nice binder. It’s now my go-to reference.

Using my new reference I traced the 3 wires from the O2 sensor. The sensor wire goes directly to the engine management computer. The 2 heater wires do not. One wire goes to the fuel pump relay and the other wire goes to ground to complete the circuit. So this confirms that the computer doesn’t receive any feedback from the heater and the O2 sensor DOES still work with the heater disabled, it just takes longer to heat up and go to the closed loop (real time information) mode. I would imagine this feature was primarily designed for cold weather operations.