Liam Dertinger

Hello all just got new headers from EM and they look great. Got a 2.5 inch straightpiped exhaust custom made to mate with the new heads. My question is about o2 sensors. There's 2 bungs on the heads that need to be drilled out. From what I'm reading on the 94 there's 3 total sensors but the downstream one is useless. When installing the exhaust how should I go about the sensors? Should I buy dummy o2 sensors for the downstream, upstream, both, or neither. I do not have emissions testing just want to avoid the CEL.

jv9999

If you want to avoid the CEL on a stock tune you will need all three to be operational.

Phobos84

He can just use an O2 SIM in place of the rear O2. They work great. You can't buy them on eBay anymore. But you can put one together yourself for around $5.

Liam Dertinger

So I'm seeing dummy o2 sensors for around 15 bucks but I need to drill and tap a space for it. The car will be dyno tuned after since I'm also removing the EGR and have added a few bolt on mods. Do I need to use a dummy o2 spacer on the pre cat sensors or just on the one post cat sensor?

Phobos84

post cat. The pre cat sensors are for fuel adjustment. The rear or post cat O2 is just for emissions.

The way I've always done it is to leave the rear O2 in place. Then I put an O2 simulator inline in the harness. Basically it uses a 555 timer to send the signal to the ECU and that keeps the CEL off. Nice thing about this is if you do have to get the car inspected it looks completely stock because the rear O2 is still in place. The signal simulator has 3 wires. 12V ignition, Ground, and signal to the ECU.

This is the one I use. I don't use the LED's. Instead I add a 47K ohm resistor to R3 for 147K ohm total for R3.


R1 100 K Ohm
R2 1 M Ohm
R3 100 K Ohm
R4 10 K Ohm
C1 4.7 uF Electrolytic
C2 22 uF Electrolytic
D1 1.7v@20mA LED
D2 1.7v@20mA LED

Liam Dertinger

So I'm putting an entirely new exhaust in the car and there's no spot for any o2 sensors so I have to drill and tap 3 holes. No worries about inspections needed. Do you have a link to the sim you're talking about?

jmgtp

I haven’t run a rear o2 sensor in my 94 for many years, it has never thrown a code for it. Your results may vary, but I don’t think the PCM is doing much if anything with that rear o2 on the OBDI cars. So you may just want to ditch it and not spend on a simulator. I just taped up the connector to keep it clean.

Phobos84

That's a good point. Try it without the rear O2 and see if the light comes on. If it does you can use the O2 simulator that I posted the schematic for to keep the light off. I'll see if I can find a link that has some better directions.

Found it

https://www.chippernut.com/blog/o2-simulator#/