Failed Emissions: EGR & Oxygen

Hi everyone. I am not a car guy. This is the step-by-step explanation of the problem.

1) Check engine light has been on for months.

2) Emissions testing indicated these two issues:

-(a) 02 Sensor Circuit No Activity Detected

-(b) EGR Flow Insufficient Detected

3) I had Autozone read the codes again and choose to replace the EGR Valve and both Oxygen Sensors. Also my buddy helped clean out the EGR in/out tubes with carb cleaner and air. All passages are clean and wiring looks to be good.

4) Check engine light is still on and reads the same problems of EGR insufficient flow and oxygen sensor is not detected.

What is the next step???



Maybe you will just need to reset the CEL.

Other than that, you need to remember that a code that indicates an O? sensor means that it is not getting the expected signal from an O? sensor. It may be that the sensor is bad, but they don’t put that light there just to test a sensor, they put it there to test the exhaust. Maybe the sensor is just fine, but the exhaust has a problem.

Now before going any further, how about telling us what make model year of car you have and how many miles on it and list the actual codes in the format [P0123] as well when the last time the plugs, wires, and air cleaner were replaced and list any recent other issues with your car.

Mr Meehan

I know that you know a lot about cars, but my understanding is that most modern vehicles use heated Oxygen Sensors. That results in two sets of DTC codes for O2 sensors – one set has to do with whether the heater has brought the O2 sensor output up into the proper voltage range within some specific time. The second set has to do with whether the Oxygen levels being reported once the sensor starts working are reasonable.

“No Activity Detected” sounds like the first kind of problem

But you are right, It’d be nice to know the specific codes as well as the brand, model and engine.



Did you or your buddy do anything to clear the codes that were set before you fixed the car? They will eventually clear on their own – probably. But my understanding is that how long “eventually” is depends on the idiosyncracies of the car computer. You can clear the old codes with most scan tools, and in most cars by pulling a fuse for 30 seconds or so. But identifying which fuse to remove often is not all that easy.

The “no activity from the oxygen sensor” could mean the signal is not getting from the oxygen sensor to the engine computer. It could be a broken wire, even.
If you want to check out the sensors, you need to be able to use a digital multimeter and be able to follow the checking procedures in the repair manual (like Chilton’s or Haynes). If you can’t, it will be cheaper for you to let an able mechanic check them out.

Car specs:
96’ Chevy Beretta v6 3.1L
169,000 miles

Codes for my current emissions test failure:
[P0140] O2 Sensor Circuit No Activity Detected
[P0401] EGR Flow Insufficient Detected

Plugs, wires, air cleaner were swapped out last summer.

*The car slightly runs a little rough on freeways.

What is reset the CEL?
I had the battery disconnected for up to 20 minutes.

If the exhaust is a problem, what would it be?

Thanks for the help.

To vtcodger2:

My advice was that if I had the battery unplugged for a while and even touched the connectors together it should reset the computer. The battery was not attached for up to 20 minutes each time a new part was installed and when we cleaned the in/out for the EGR valve connection.

I have a manual from Haynes.

I’m thinking I should just replace fuses as a simple cheap step.

Any more thoughts???


Yeah, disconnecting the battery should clear the codes.

There are probably at least two Oxygen Sensors – one after the catalytic converter and one, maybe two if you have a V engine, before it. P0140 is the downstream sensor, the one toward the muffler from the catalytic converter. That’s the one you replaced, right? You can try tracing the wiring, but I imagine that it will disappear into a wiring harness. The Haynes Manual will give you the wiring color code, but you probably don’t want to get mixed up in punching pins into wires and measuring voltages unless you have three arms and are used to working with electronics. What is supposed to happen is that when you turn on the ignition, a voltage is put on one of the wires that starts up a heater in the O2 sensor. As it heats, the voltage on the sensor wire will slowly change to its initial value. Once everything is working right, the voltage on the O2 sensor wire is a function of how much Oxygen is in the exhaust stream after the catalytic converter has burned all the unburnt fuel. It shouldn’t change much, but if it did, that’s be a different DTC code. I’m pretty sure that the code you’re seeing is that the O2 sensor is too slow to reach its expected initial value. That’s either bad connection, a dead O2 sensor heater, or an elderly O2 sensor.

If you can’t find a loose connector, burnt wire, etc, you’ll probably need to find a mechanic with skills that go beyond parts hanging.

The EGR code you’ll have to talk to someone else about. What EGR does is open a path that allows some exhaust gas to be fed back into the input gas air mixture in order to make the engine run a little cooler. (No, I don’t really understand that). Anyway, an EGR valve stuck closed or similar problem would (I’m told) have no impact on engine performance, but would set the 401 code you are seeing. An EGR valve stuck open causes rough idle and would presumably set a different code. Your buddy may be able to help on that one. I’d look into how exhaust gas flow is measured on your vehicle. Might be a problem with the sensor they are using. Or it might be something else entirely.

You don’t have adequate test equipment to check this out. You need a scan tool which can display the oxygen sensor wave pattern. The code P0140 means that, either the oxygen sensor isn’t making a voltage wave pattern, or the signal isn’t getting to the engine computer, or the engine computer is blind to it.
You could check the wires from the engine computer to the oxygen sensor with a digital multimeter. Check ohms by disconnecting the battery, disconnecting the wires at the oxygen sensor and disconnecting the connectors at the engine computer.

Check engine light on for “months”?
You “Do I need that part” months ago.
Not detected means the connect of one O2 sensor wasn’t made.
Correct OEM sensors? That is Bosch or replacement with the correct plug?
Solve the O2 problem first.
Real good chance you fried the cat.

Does the CEL clear (go off) after you disconnect the battery, then, after a period of time, come back on?? Or does it just stay on after your attempt to clear the codes? If it just stays on, sounds like you have a wiring issue, chipmunks chewing on the wiring harness, something like that…

For what it’s worth, the days of DIY auto-repair, especially with problems like this, are over.

For what it’s worth, the days of DIY auto-repair, especially with problems like this, are over.

I disagree wholeheartedly. The tools required are different. A digital multimeter is a must. And a good shop manual. But, a lot of repairs are easier with today’s OBD-II than the old days of chasing problems with sniffers, oscilloscopes, and dwell meters. Most service repair manuals include simple tests to check components before blindly replacing, which is what most DIYers do and get flustered when that doesn’t work. For anyone working on thier own car, I strongly recommend getting a Factory Service Repair Manual. These books are invaluable to correctly fixing problems.