I was asked to look at this Mitsubishi Shogun with EML on. This had already been looked at by another garage, the customer has taken it back to them twice with the same problem and they have failed to fix it despite money already spent.
These jobs as we all know can be a problem, the customers budget already spent on unnecessary parts and time by the previous garage, and the pressure is on to get the diagnosis right first time.
A challenge I like and I agreed to take the job on. After interrogating the customer I started the process, first thing code read.P0125 - Insufficient coolant temperature for closed loop fuel control
A code I have never seen before. Done a visual check and could see a thermostat and an engine coolant temp sensor had been fitted by previous garage
I decided to monitor live data from cold to running temperature, not 100% sure what i was expecting but just looking for anything out of the ordinary.
Live data showed engine coolant and air temp as normal, but scrolling down the data I noticed Bank 2 sensor 1 only reached a maximum of 0.24v during the whole warm up period which was about 30 Mins
This was the only thing that stood out to me,it obviously was not right, now I needed to find out what was happening and could this be related to my fault code.
The connection block for both O2 sensors was fairly accessible at the bulk head, resistance across the heater circuit was 7 Ohms, once engine had cooled down again I connected the scope and used all four channels. Measuring signal, supply, duty cylce for heater and heater current, earthed the scope at the sensor
Every thing except the signal was normal, as serial data showed, bank 2 sensor 1 was around 0.2v.
I wanted to see what the sensor would do with a snapped throttle, so I freed up the channels and I introduced bank 1 sensor 1 with bank 2 sensor 1 and throttle position, snapping the throttle we would expect to see a rich state through out the WOT (Wide open Throttle)
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As pointed out the green trace is bank 2 sensor 1 on wide open throttle the sensor goes sub zero, something that I have only seen a few times.
This would have never been discovered with out the pico, Maybe this is why the other garage struggled.......
Don't get me wrong here this was a complete learning curve for me as I have never experienced the fault code before, and didn't really have a diagnostic structure in place. It was a kind of earn as you learn repair, either way, new O2 sensor was fitted and then carried out another WOT test with both sensors, and proved both sensors at a rich state during the WOT and no sub zero voltages.
Fault code cleared and extended road test performed, connected scan tool, no fault codes present or pending.
The pico was infallible in this case, it just shows a bit of time and the right tools you get a fix and no unnecessary parts fitted...