Vehicle diagnosis is like an onion it can have many layers and make your eyes water.
I was called to a garage to check out a Toyota Rav 4 they had in for diagnosis. The vehicle was very flat hardly able to pull away under its own steam. The fault history had been cleared and the only fault remaining was O2 sensor bank 2 sensor 1. Unsure which sensor is bank 2 sensor 1 they were going to order a pair. The price soon put a stop to that plan. This vehicle is fitted with wideband sensors which are much more expensive than the narrow band type. I did a quick inspection of the wiring, and quickly tested the heater circuits using a scope. Bank 2 sensor 1 had a open circuit heater. A new sensor was ordered, how ever this was unlikely to cure the flat running problem.
I had a quick look at the live data, and noticed the air mass was under reading at WOT. I checked this with a scope and sure enough it was only hitting 2.7V on snap acceleration. It should be closer to 4.5V. I checked the supply and earth connections both were fine. So a new air mass meter was added to the order.
After both components had been fitted and the codes cleared the vehicle displayed normal driving characteristics. As is often the case the fault code and mil light did not relate to the symptoms. The customer wanted the light switched off, so I guess that renewing the O2 sensor would of fixed that, but the air mass was causing the flat running. This is typical of engine management faults when you will be presented with multiple codes, or multiple faults or even both. You have to peel these layers back to reveal the fault(s). The cost of the parts can make your eyes water.
