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A 2011 Yaris diesel drove normally but lit up the dash, refused to talk to a scan tool and kept dropping its rev counter to zero. The clue was the steering wheel. The cause was the engine ECU not grounded to its case.
By Rootes Motors
A 2011 Toyota Yaris with the 1.4 D-4D diesel. It had warning lights on the dash and our scan tool could not communicate with it through the OBD port. Every now and then the rev counter dropped to zero and came back.
The odd part was that the car drove normally. Nothing about the way it ran matched the dash.
No communication means no fault codes, so this was a meter-and-wiring job from the start.
We began with the power and ground side. The first readings looked alarming: continuity between battery positive and the engine casing, and 12 V between the battery negative terminal and the casing with the negative lead off. With the battery out of the car, the continuity reading lasted a second or two and then broke.
Those readings were a dead end. Once the supply side was rechecked, the voltages were fine, and we moved on.
The next day we found a new symptom. While the steering wheel was being turned, the rev counter and the OBD connection came alive for a moment.
We did not suspect the steering itself. Our reading was that the movement was briefly completing a connection that should have been permanent. That narrowed the search to grounds and connectors, not modules.
The engine ECU was not grounded to its case. That was the fault.
It fits everything we saw. The engine itself ran, so the car drove normally. What failed was the ECU's conversation with the rest of the car: the scan tool, the dash and the rev counter.
If your car is showing a dash full of lights but drives as normal, our diagnostics page explains how we approach it.
Questions?
Happy to talk it through