Vehicle details: Toyota
Symptom: MIL on,
Power loss
Author: Steve Smith | Pico Technology

Toyota | Loss of power P1251

One would hope that the majority of garages throughout the world would agree that a diagnostic scan tool is simply not enough, but this would not appear to be the case.

The PicoScope journey forms a typical yet simple example of how PicoScope sits alongside any scan tool on the road to a successful diagnosis.

The flow chart demonstrates how you can far exceed customer expectations by using PicoScope during the diagnostic journey, not only ensuring a first-time fix, but protecting all parties involved with physical before and after test results (not to mention increased workshop efficiency as a desirable side effect).

Let’s take a look at what is all too often a repeated scenario when you leave PicoScope out of the diagnostic journey.

Our customer arrives with an engine management light (MIL) illuminated and complaining of power loss. You attach the scan tool and find a fault code of P1251 with the relevant freeze frame data (P1251 Turbo overboost). A basic inspection confirms that hoses are OK and finds no connection issues. You erase the fault code and road-test the vehicle, finding no concerns. You charge the customer accordingly but he returns the following day with the identical symptoms mentioned above.

Once again you connect the scan tool and find code P1251. You examine the live data during a road test and find no issues. Given that the same fault code has returned, you decide to replace the turbo boost control vacuum switching valve (VSV) complete with hose network, and the return the vehicle to the customer. Once again the vehicle returns to the workshop with fault code P1251 stored and loss of power. As a result of intense customer pressure from loss of vehicle revenue, you reluctantly opt to install a replacement turbo charger without evidence to confirm if the turbo is in fact the cause of the original code P1251.

With heart in mouth you release the vehicle to the customer in the hope that the fault has cleared. A week has elapsed with no sign of the customer and so it would seem the fault has cleared and the issue is resolved. This is diagnosis by the seat of your pants with no evidence or documentation to warrant such a repair, resulting in a customer who now questions your professionalism, accompanied with unnecessary workshop loading that would most certainly impact upon the daily bookings for existing customers.

Does this sound familiar?

Now let’s take the same diagnostic journey when PicoScope is included.

Our customer arrives with an engine management light (MIL) illuminated and complaining of power loss. You attach the scan tool and find a stored fault code of P1251 with relevant freeze frame data (P1251 Turbo overboost). A basic inspection confirms that hoses are OK and finds no connection issues, but in addition you now attach the PicoScope with a pressure transducer to evaluate boost pressure actuator vacuum control, the control signal to the boost control VSV, the MAP sensor signal and turbo boost actuator position sensor.

With all the above inputs in place, you road-test the vehicle in an attempt to reproduce the symptom. Although the fault code does not return, you gather clear evidence confirming early stages of a momentary overboost condition not indicated by the ECU detection strategy but clearly visible using PicoScope.

The image above demonstrates how monitoring the input to the boost control VSV and corresponding vacuum in real time with parallel not serial testing (scan tool), we can see live events as they happen and not the ECU’s interpretation and presentation of events.

Looking closely at the vacuum control signal supplied to the boost control VSV we can see how the ECU sent the command to reduce boost and the correct response was followed by the boost control VSV given the fall in vacuum level to the boost control actuator. However, MAP pressure remained high while the boost actuator position sensor indicated only half travel.

From the above captured data we can conclude that the ECU is functioning correctly as it sent the command signal to reduce boost. We can confirm that the wiring to the boost control VSV and the valve/hose network is functioning correctly as reported by the pressure transducer, leaving only one possible explanation for the failure in the reduction of manifold pressure: turbocharger variable vanes sticking.

Introducing the PicoScope during the diagnosis gave us the ability to monitor command and response times for turbocharger control, so identifying the true cause of fault code P1251 where the engine control unit would have been monitoring the turbocharger control under a specific strategy requiring an overboost condition for a predetermined time period and count.

Here you have irrefutable evidence to present to the customer and the relevant warranty companies that the turbocharger in question has developed an intermittent fault resulting in overboost.

With confidence, you replace the turbocharger resulting in minimal workshop effort. You then return the vehicle to service in a timely fashion leaving a lasting positive impression upon the customer.

While the scan tool remains essential, the PicoScope is critical to a successful diagnosis and completion of the diagnostic journey.


2 comments | Add comment

November 07 2017

Hi-my doing the same-did you fix it-we have changed-turbo-egr-alternator-airfilter sensor-oil sensor-still 1251 comes back and limb mode-if you can help anyway that would be great

James Millard
May 21 2017

My Hilux was going into limp mode due to this code stating stepper motor intermittent circuit fault. It has just had a new turbo at Toyota and has come back going into limp with the same code even more than before. Any longer run driving normally…....
They are trying to look for answers like the remap or exhaust but it started when it was stock. I feel like they are trying to fob me off but after the hefty bill no chance. Seeing as it’s worse…....... even tried testing the fuel.

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Case study: Loss of power P1251