A heated issue

A choice between using an aftermarket and OE component gives Barnaby Donohew a headache

Published:  09 January, 2014

By Barnaby Donohew

We at T J Lobb have a policy of fitting only genuine OE exhaust gas sensors to EOBD compliant vehicles - in many cases, the systems in these vehicles introduced tighter operating tolerances. Fitting genuine components helps us avoid complications when parts are not manufactured to meet these tolerances. However, as the vehicle's model year pre-dated 2000 and against my own better judgment, I decided to try an aftermarket component.

A fault code memory scan revealed the same P0031 trouble code. A PicoScope was employed to measure the heater circuit voltage at the sensor connection and the circuit current. Examinations of the voltage revealed a stable signal with full battery voltage during the on phase and approximately 0V during the off phase. With a stable current signal, the measurements ruled out faulty wiring or ECU.

Repeated road testing showed that the engine management light would return at the moment that a heating phase terminated. The scope waveforms suggested nothing out of the ordinary; however, the ECU had determined that the circuit was operating incorrectly at some point up to or including that instant. Attention returned to the operating characteristics of the oxygen sensor and it was decided to try a genuine OE unit.

Lo and behold, after fitting a genuine component and during all subsequent road tests, the engine management light stayed off.

Although the article here compares sensors produced by one brand, it is likely that aftermarket sensors manufactured by other brands might have also been rejected by the vehicle's system. A 'Google-diagnostic' search reveals many reports regarding its particular sensitivity to non-genuine sensors. Likewise, many technicians will be familiar with similar cases relating to other vehicles. The point is that EOBD has been mandatory for 13 years and yet the aftermarket appears not to offer a solution to the described problems, effectively leaving independent technicians little choice but to fit genuine components.

It should be noted that the figure provided shows a snapshot of the heater circuit's activity: various attempts were made to make repeatable observations regarding each sensor's characteristics but there was too much signal variability to come to any firm conclusions. Consequently, it is thought unlikely that scope testing of the sensors would offer a technician any practical insight as to whether or not a sensor is fit for purpose.

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