Apparently, the engine compartment fuse box is setup for a number of car models. Thus, the fuse numbers and fuses seem to not correspond. Does anyone know which fuses power the heat for the 02 sensors? Thanks

 

I'd use the Electrical Guide (and anything you can also glean from car handbook).

 

That's the problem, that info isn't in there. I'm chasing an 02 sensor that won't "smog check" and one of the possible reasons is a bad fuse.

 

You wanna throw us old dogs a bone? Is this an '08 V8 NA per your profile?

What fault code(s) are you getting? I'm guessing somewhere between P0031 to P0058 for the O2 sensor heating circuits? Are you getting a single code? Multiple?

Wiring diagrams and OBD II codes are here:

JagRepair.com - Jaguar Repair Information Resource

Wiring diagrams are at the top section. 2006 is the latest wiring diagram, but I believe that covers all models 2006+. OBD II codes are in the next section labeled (wait for it) OBD II codes. The latest info there is 2003.

In the wiring diagrams, the O2 sensors can be seen in figure 03.3. Follow the power sources back to figure 01.7. In the front power distribution box, fuse F20 powers one pair of sensor heaters. F35 powers the other two. If all 4 heaters are being flagged, check relay R2.

Fuse locations can be seen on page 14 of the wiring diagrams:




Does that match what you have on your car?

 

Thanks, Karl.

I figured I said that just without that amount of hand-holding...

 

As car in profile says, yes 2008 4.2 NA.. No that image does not match my car. But it sort of does match what's in my manual... Very late car if that matters. But let me see if I can extrapolate. (No faut codes. Yet) Thanks!

 

Can you please elaborate?

Your previous statement: chasing an 02 sensor that won't "smog check"

I'm thoroughly confused. Has the car failed an official smog check at a test facility? If so, I'd think you'd have a corresponding OBD II code.

Or is this something only on your scanner? Mine has a feature that lets you prepare for an official smog check and let's you know if all monitors have been set and no fault codes are present. In other words, the car will pass once the official test equipment is connected to the OBD port.

 

OK, what I mean is my 02 sensor won't "clear" with a ton of driving. But there is no code (on my scanner). Which is why I thought I'd start with the fuse, then to the harness before I replaced the sensor. What I read (somewhere) was that if an 02 sensor is "marginal" it may not throw a code, but also it may never clear. I'm just going to pull fuses and see if any are bad (multi-meter test) then move on to inspecting the harness. Thanks!

 

Okay, we seem to be making some progress. From what I understand now, you're saying the O2 sensor monitor won't complete.

Could it be the sensor heating system? It won't hurt to check the fuses, but there are specific codes that will be generated if the heaters are not working. I don't know all the details, but would hazard a guess it's based on amperage draw. The main purpose of the heater circuit is to bring the sensors up to operating temperature quickly from a cold start, so the control system can switch from open loop (engine cold) to closed loop (engine warm). Once warmed up, the heater circuit should have nil effect on the actual sensor action.

Just for giggles, did you see the official O2 sensor monitor cycle? This was on page 5 of the OBD II code cheat sheet linked in my previous message. Straight from the horse's mouth:

HEATED OXYGEN SENSORS MONITOR DRIVE CYCLE
Upstream (Universal) oxygen sensors:
1) Engine OFF; cooling fans inoperative > 20 seconds.
2) Start engine and bring to normal operating temperature > 82 °C (180 °F).
3) Drive the vehicle between 3000 – 4000 rpm in 3rd gear at a steady speed. Lift foot completely off accelerator and coast to a stop within 30 seconds. Do not touch accelerator pedal for 4 seconds after coming to a stop.
4) Repeat step 3.
5) Idle engine for 11 minutes.

Downstream oxygen sensors:
1) Start engine and bring to normal operating temperature > 82 °C (180 °F).
2) Drive the vehicle steadily between 48 – 97 km/h (30 – 60 mph) for 10 minutes.
3) Drive the vehicle above 3000 rpm in 3rd gear at a steady speed. Lift foot completely off accelerator and coast for 30 seconds.

Oxygen sensor heaters:
1) Start engine and bring to normal operating temperature > 82 °C (180 °F).
2) Idle engine for 3 minutes.

End copied text. Sounds like a pain to do it exactly as described. But if nothing else is working, it may be worth trying.

 

Thanks KR -- I have done something similar -- but yeah, I will check heater fuses as well though I read they'd throw codes. And will redrive as per your instructions!

 

Any unset monitor means related codes can't flag (car doesn't have data it can trust).
I'm unclear exactly which O2-related monitor you mean.

It may be worth looking at OBD live data for clues (e.g. using an elm327).

There are lots of gory details of what's going on in the attached

 

Thanks for this!

 

So I got AI involved:

The real problem is none of these diagrams match my car. They match my manual, but neither match the carThat’s a really important distinction you’ve noticed — and you’re not alone. With Jaguars of this era, the manual and lid diagrams often don’t match the actual fuse box installed in the car. Here’s why:

🔎 Why the mismatch happens

• Shared fuse box molds: Jaguar used the same physical fuse box design across multiple trims (3.0 V6, 4.2 NA, 4.2 Supercharged, diesel). The lid diagram shows all possible fuse positions, but your car only has the ones relevant to your engine and market.
  
• Mid-year revisions: In 2007–2008, Jaguar changed fuse assignments during production runs. Manuals printed early in the year don’t always reflect the later wiring harness/fuse layouts. (Except mine IS a 2008)
  
• Market differences: US-spec cars sometimes had different fuse ratings or positions compared to UK/EU-spec cars, especially for emissions-related circuits like O₂ sensors.
  

✅ How to resolve the mismatch

1. Ignore the lid diagram — treat it as generic.
   
2. Use the manual for fuse ratings — that’s correct for your trim (20A for O₂ sensor heaters).
   
3. Physically trace the O₂ sensor heater wires:
   
   • The heater wires are usually white or gray in Jaguar harnesses.
     
   • They run from the O₂ sensor connector back to the engine bay fuse box.
     
   • Using a multimeter, you can confirm which fuse slot actually supplies voltage to the heater circuit.
     
4. Check all 20A fuses in the engine bay — even if they’re labeled differently, one of them will be powering the O₂ sensor heaters.
   

⚠️ Practical tip

If you want absolute certainty, the quickest way is:

• Pull one 20A fuse at a time in the engine bay.
  
• With the fuse removed, check for voltage at the O₂ sensor heater connector.
  
• When voltage disappears, you’ve found the correct fuse for that sensor circuit.
  

Would you like me to map out a step-by-step “live test” procedure with a multimeter so you can positively identify which fuse in your actual car is powering the O₂ sensor heaters, regardless of what the manual or lid says? That way you’ll have a definitive answer tailored to your specific fuse box.
right...

 

If you do that, see how it matches the Electrical Guide.