Hi all,
I have a 2002 MX-5 with a 1.8 VVT engine (the SVT model). Back in July I got an EML light & noticed the car was running rich so I read the fault codes and got the code P0420 - “Catalyst System Efficiency Below Threshold (Bank 1)”. Looked at the live data & was getting nothing from the rear O2 sensor so I’ve put a new genuine one in from MX-5 Parts. (The internet seems to suggest it would either be an O2 sensor or a new cat).
Anyway, now I am getting the code P0037 - “H02S Heater Control Circuit Low (Bank 1, Sensor 2)”. I did get a voltage off of the rear sensor when I initially put it in for ~30 seconds but since I have got nothing out of it. I’m wondering if it’s worth buying another O2 sensor or maybe the problem lies somewhere else in the loom? Is there anyway of checking the sensor works?
Any advice would be greatly appreciated.
David
Might be worth refitting the original sensor to se if the new DTC clears. I can give you some tests to carry out this evening.
When you say you were getting nothing on the rear sensor what was there any value?
What are you live data readings showing? A good sensor should show the voltage switching between 0 an 1 volts.
P0420 is usually the front sensor.
Also worth checking for air leaks before the sensors or around the inlet side as well.
Is it the correct cat as well? 2002 should have a type approved catalytic converter fitted. Fitting a non type approved one will throw the EML on every few hundred miles due to the lower level of precious metals in the core.
I did try fitting the old sensor back yesterday. Didn’t read the codes but was not getting a voltage from it still & the EML was still on. Can have another go tonight if the floor is not wet! Am working outside. I’d be happy to try some tests though.
I was getting a constant 0V
Live data readings are giving me a constant 0V. The front sensor is good it’s voltage moves around.
I couldn’t hear any air leaks. But I can have another check.
The exhaust system is fairly new unfortunately I don’t know whether the cat is an oem one I have no paperwork for it.
Sounds like a duff sensor. However, check the wiring where it goes under the seat for damage (same as the plug) and also fuses. As for the cat, you need to get underneath and look for any part numbers or EC/ECE markings etched into it.
Who fitted the system? Previous owner or garage? Its possible the wrong one (cheaper non type approved) has been fitted hence the issues you have got now.
I’ve recently had sensor issues related to the exhaust on my not MX-5 vehicle.
Had a dpf deep clean done but still getting warning messages.
I tried putting Wynns system cleaner in the fuel about a week ago and no problems since (so far!) 
For the below, I’m assuming that the last 6 digits of your VIN are higher than 204962.
For P0037, you need to check for voltage at the Black/White wire of the HO2S plug on the vehicle wiring harness side of the plug with the ignition on.
This wire is supplied with battery voltage from the ENGINE fuse in the fuse block behind the dash board, I’d expect the fuse to be OK to be honest, you’d have a lot more problems if it wasn’t, but check that it is and that there is battery voltage either side of it.
The ground side of the HO2S heater is the White/Black wire. I’m not sure under which conditions the PCM switches the heater on and off, but the change point seems to be at 70*C engine coolant temperature. Check the voltage at the White/Black wire with the engine cold, then see if it changes as the engine approaches normal running temperature. I’d guess that you’ll see zero volts on the ground side with a cold engine (heater on) and 12 volts with a warm engine (heater off). The PCM is looking for this change in voltage when it switches the heater off to determine that it’s working. If it doesn’t see it, that’s what triggers the code (voltage remains low).
The resistance across the HO2S heater should be approx. 15.7 ohms (with it disconnected and checked across the corresponding 2 wires in the HO2S connector).
I hope the above all makes sense and you can take some measurements from it.
Hi all just thought I’d give you an update as I had a go with the sensor tonight.
First of all I decided to take a good look at the connectors on the sensor, they look okay? I’ve attached a picture so please do give your opinions.
I had a go at tracing where the wires go up into the dash but they disappear off into the abyss and there’s only so much of my car I can take apart in the flat car park before it goes dark.
Had a good look around the car for where things might be earthing, found some earths to the chassis under the dash on the drivers side. Gave them a wiggle, they seem to be connected and in good condition. Also found the earth on the back of the engine (I’m not sure if either of these are related to the oxygen sensor but they both looked alright).
After that I decided I’d done enough digging around and decided to plug the obd2 scanner in and clear the codes. Then I turned the car on (sensor connected). Initially no voltage (like before). Let the car heat up while I was putting everything back together again & then drove up & down the flat car park - car not allowed on the public roads at the moment. When I parked the car up there was a signal from the sensor (yay) and no EML. Maybe it was just a dodgy connection? I’m not sure.
As the car had been running rich initially then I couldn’t tell whether it had stopped running rich, still got some petrol in the exhaust so couldn’t tell if it had stopped running rich. Tomorrow I will turn the car on again & let it heat up - lets see what happens (hopefully it doesn’t run rich).
If the problem decides to come back I’ll try what you’ve suggested Robbie.
Thank you everyone that’s helped so far I will come back with more updates tomorrow.
Had a crack at measuring the outputs from the plug today. Got 14V from the black/white wire & all the other wires were 0V (on the loom side).
On the sensor side (not plugged in) was getting ~0.4V from the wire that corresponds to the black/red wire on the loom side. All the others were 0V.
