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Hi, I have a 1994 MX-3 GS 6 cylinder. I was wondering if one of my O2 sensors may be going bad.

Here's the symptoms:

Poor gas miliage
High Idle when starting, then after it runs awhile it settles down to 900/1000 RPMs

CEL comes on when the car coasts or when decreasing acceleration. CEL goes off when I step on the gas.

I get the codes 15 and 24

Smell of exhaust is noticible but not overwhelming.

Is this the O2 sensor, and how do I test it?

15- Oxygen sensor or circuit
24- Rear 02 sensor inversion error

However, the exhaust smell confuses me. Are you smelling it in the cabin? It would seem like the codes are being thrown because of an exhaust leak early in the system. Maybe an exhaust header gasket or something before the resonator. The mx i had before the one I have now had a leaky fuel rail and I swear exhaust fumes were coming out of the leak in the intake right where the fuel injectors are. But, if that were your case it would also be throwing 17 and quite possibly 25? anyone else agree?

Does the detonation sound seem to be coming from the muffler or the engine. Easiest way to find an exhaust leak to me

Could be as simple as vacuum leak somewhere. Check online manual for proper diagnosis.

icajewler wrote:15- Oxygen sensor or circuit
24- Rear 02 sensor inversion error

However, the exhaust smell confuses me. Are you smelling it in the cabin? It would seem like the codes are being thrown because of an exhaust leak early in the system. Maybe an exhaust header gasket or something before the resonator. The mx i had before the one I have now had a leaky fuel rail and I swear exhaust fumes were coming out of the leak in the intake right where the fuel injectors are. But, if that were your case it would also be throwing 17 and quite possibly 25? anyone else agree?

Does the detonation sound seem to be coming from the muffler or the engine. Easiest way to find an exhaust leak to me

It seems to be coming from the exhaust, the car sounds "throaty" if you know what I mean.

is there a link for checking an exhaust and a vacuum leak?

Thanks for your help!

You have bad oxygen sensors. Replace them to improve gas mileage. The codes are telling you exactly what is wrong.
Code 15 is the left side oxygen sensor not sending a proper signal after the engine warmed up (specifically the signal did not exceed 0.55V 100 seconds after engine speed exceeds 1500 rpm)
Means: You have a bad sensor on the left side (near radiator)
Code 24 is right side oxygen sensor not changing in response to the ECU adjustments.
Means: You have a bad O2 sensor on the right side (near firewall)

Idle is exactly as intended (high idle until the engine is warm).
Poor fuel economy and exhaust smell may be due to running too rich because the ECU does not have good O2 sensors to fine tune the air/fuel ratio.
Throaty exhaust sound might be a leak or more likely your muffler is rusted out inside.

Sometimes an exhaust leak will throw an oxygen sensor code even if the sensors are actually "good". That only happens with a down stream oxygen sensor (after the converter) that is monitoring the health of the catalytic converter. This is not the case for these codes.

I think it's odd that both sensors are bad at the same time. I replaced the distributor and timed the engine, so I disconnected some hoses to get to these areas. Looks like they are all intact to me, but I think maybe something is loose.

So, I checked the codes again: I guess I was wrong, I'm getting code 16 (Exhaust gas recirculation (EGR) system) and code 24.

Could be a leak in the header with the egr appendage? If budget allows, replacing o2 sensor might not be a bad idea. Check over some of the hoses or electrical connections that may have been in the area of work. The easiest thing to troubleshoot I think would be making sure the intake piping doesn't have any leaks. These k series are very particular about air flow. For the possible exhaust leak, I used a spray bottle with soap and flashlight (for those dark areas) and sprayed the exhaust pipe connections until I saw the bubbles. Unconventional because of the 'unprofessionalism' about it. But I'll use what works over takes too long and costs too much

I would go ahead and replace both of the sensors with Denso OEM brand. Even if an O2 sensor isn't throwing a code doesn't mean it's functioning as it's supposed to.
an improper burn could lead to a smell. When I replaced my o2 sensors it did affect the scent of the exhaust. It was milder afterwards.

It sounds like you should replace the right side O2 sensor.
Tip: There is a universal Denso (this is OEM brand) sensors for around $30 each through online sources (rockauto, ebay, amazon). I used Denso 234-4209 for both. The wire colors should match up.

The EGR error is related to the position feedback sent by the valve. It is a vacuum operated valve with the electrical connection sending a position signal back to the ECU. Check the vacuum connections and to be sure the vacuum is routed correctly through the 2 solenoids that control position (EGR Vent and EGR vacuum). All must be connected properly to work. If vacuum and electrical connections check out then you might consider removing and cleaning the EGR. Sometimes the internals of the EGR get gummed up by carbon deposits from sitting in the flow path of exhaust gases.