2002 VW GTI 1.8T - O2 sensor fault code only when exceeding 70mph

Question

During my last roadtrip when I was driving at about 80mph on the highway (75 speed limit) a pressed the brakes abruptly after being cutoff by another vehicle, then my MIL came on. When I checked the fault code with VAGCOM it said:

17522 - Oxygen (Lambda) Sensor: B1 S2: Internal Resistance too High

P1114 - 35-10 - - - Intermittent

(2002 VW GTI 1.8T, 67k miles odometer)

I reset the code and the light stayed off, but ever since that incident, now when I reach about 70mph on the highway (and sustain that speed for about 30 seconds) the MIL comes back on again. After a day or two the MIL turns off automatically and remains off until I exceed 70mph again (same fault code).

I have reproduced this problem 3 times. As long as I stay below 70mph on the highway, then NO fault code appears. I don't notice any strange behavior or decrease in performance, but I'm now staying below 70mph until I can figure this out.

Is this more likely an issue with the wiring/harness or the O2 sensor itself, or possibly something else entirely?

Thanks for the help.

Answer 1

It is most likely that this sensor has failed.

The question is why does the MIL only come on above 70mph. The answer lies not in the sensor but how and when the sensor is tested by the PCM. Every sensor (and actuator) in the engine control system is tested by the software, but not all the time. Each sensor has its own set of test entry conditions; each OEM will vary when and how each is tested. Regulatory rules define test conditions for a few of the parts in this system but not all, so the OEM's are free to choose when they perform the tests. Their primary goal when designing the software is to keep the MIL off whenever possible while having the system satisfy the minimum regulatory requirements. The solutions they have chosen over the years vary greatly.

So, in the case of this OEM, with software written at this time, they test resistance on this sensor at or above 70mph.

The next question is why does the MIL go off after a few trips?

The regulations allow the light to be turned off after two trips in which the part passes an on-board test. There are two possible cases to explain this:

1) The resistance test is run at power on and it passes that test. This type of circuit test is most commonly done at power on. Or

2) VW wrote the code incorrectly. The CARB regulations were extensively rewritten for the 2004 model because of significant errors such at this is anomaly implies.

Answer 2

I haven't been able to find any solid conclusions on the VW forums regarding the root cause for this behavior, but there are some clues as to what could be causing this from the diagnostic code.

Fred Wilson's answer remains a distinct possibility but I struggle to reason with why the engine computer would choose to throw a code/MIL for a bad sensor on a speed-based condition.

To me, a far more plausible explanation for what you're observing is that it is an intermittent issue due to loose wiring, for the following reasons:

• "Internal resistance too high" might be interpreted to apply to the situation where the circuit for the O2 sensor signal becomes open (close to infinite resistance)

• It is plausible that the loose connection could have been induced by heavy braking, which would explain why you see the problem only after your sudden-braking incident

• If the vehicle surpasses a certain speed, it may load the O2 sensor connector or wiring in a way that the circuit becomes open (mainly wind, aerodynamic drag, vibrations)

• I wouldn't expect an O2 sensor itself to work intermittently. It's either giving a signal or it's kaput

Suggestions

• Confirm that you have good signal coming from the O2 using VAG-COM or a scan tool. Given the intermittency of the problem, I don't think you have a bad sensor

• Inspect the wiring harness and make sure that the connections are solid. If you don't see anything remiss, hit both male and female sides of the connectors with some electronic contact cleaner before reconnecting them

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It can be tough to diagnose such issues over the Internet, but based on the information provided I believe this would fit the symptoms like a glove. Hope this helps.

Answer 3

Because this issue is speed dependent, I'm almost certain that it is related to exhaust gas temperature. Paulster2 suggested checking whether the issue happens at a lower gear. However, then the load on the engine is lower, so there are less exhaust gases. It may be the case that the issue does not happen at a lower gear, therefore. But a good idea to check whether it occurs at a lower gear, anyway.

I'm almost certain this is an indication of the oxygen sensor failing due to the heating resistor starting to fail. The exhaust gases should mainly affect the temperature of the heating resistor, not as much the temperature of the wiring. So, I would replace the sensor to see if it remedies the issue.

If you're on a tight budget, replacement can be postponed. The sensor failure should not cause catastrophical troubles to the engine, and most cars have two sensors nowadays, so if one sensor fails, the second can provide some information to the engine management system. The main problem of a sensor failing is that in theory it may increase your emissions, but since this is a heating circuit failure and occurs only at high speed, the sensor is then already warm, so I don't believe your problem increases emissions yet. But if the heating circuit fails completely, then I believe emissions will be somewhat increased.

The mandatory inspection can be failed due to the trouble code. So, drive the car to the mandatory inspection at low speed and hope the problem does not occur during the inspection, if you're planning to postpone replacement.