P1151 – “Fuel Trim System Too Lean” or “Coolant Heat Storage Tank”

By Benjamin Jerew (Contact Me)
Last Updated 2016-10-18
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P1151 Lack of HO2S-21 Switch, Bank 2 Sensor Indicates Lean (Ford) Heated Oxygen Sensor (HO2S) Lean Mixture Indicated (Mazda) Bank 1, Long Term Fuel Trim, Range 1 Leanness Lower Limit Exceeded (Volkswagen) Coolant Heat Storage Tank (Toyota)

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What Does Code P1151 Mean?

This DTC (diagnostic trouble code) is manufacturer-specific, Volkswagen, Ford, Mazda, and some other makes use similar definitions, such as:

  • Lack of HO2S-21 Switch, Bank 2 Sensor Indicates Lean (Ford)
  • Heated Oxygen Sensor (HO2S) Lean Mixture Indicated (Mazda)
  • Bank 1, Long Term Fuel Trim, Range 1 Leanness Lower Limit Exceeded (Volkswagen)
AFR Signal Analysis
AFR Signal Analysis

Oxygen sensors are used by the ECM (engine control module) to compare the amount of oxygen in the exhaust to that in the atmosphere. Indirectly, the ECM can judge air-fuel ratio and adjust for emissions. If the ECM detects that the oxygen sensor is stuck lean, that is, it is not switching from lean to rich to lean, it records this as a fault, stores a DTC, and illuminates the MIL (malfunction indicator lamp).

Toyota Prius Coolant Heat Storage Tank
Toyota Prius Coolant Heat Storage Tank

Toyota defines this DTC as Coolant Heat Storage Tank fault, specifically on Toyota Prius. To improve warm-up emissions, the Prius cooling system includes a hot coolant storage tank, basically a thermos, to feed hot coolant back into the engine at startup. This preheats the engine and helps it get to operating temperature sooner. The ECM uses temperature sensors at the inlet and outlet of the tank to verify tank function and for engine prewarming programming. If the ECM detects that the storage tank isn’t keeping coolant at the proper temperature, it will set a DTC and illuminate the MIL.

What are the common causes of code P1151?

Depending on year, make, and model, DTC P1151 may have number of causes. Here are some of the most common.

  • Ford TSB (technical service bulletin) – Ford E-150, E-250, and F-150 4.2 ℓ engines had a common intake leak at the upper-to-lower intake manifold mounting area. The problem was often intermittent and hard to reproduce. New gaskets and bolts with rubber seals were prescribed to seal the intake properly.
  • Toyota Prius – It seems that the most common cause for this problem is improperly bleeding the cooling system. Because of the way that the cooling system is routed, bubbles may form in higher parts and stop the flow of coolant. The ECM will be unable to detect a change in coolant temperature with the tank open, so it will judge it a tank efficiency problem.

What are the symptoms of code P1151?

In either case, at least with the makes discussed here, you will probably not notice any drivability problems. In the case of the Fuel Trim System Too Lean, in particular, you will probably notice a drop in fuel economy.

How do you troubleshoot code P1151?

For more information on troubleshooting Fuel Trim System Too Lean, refer to DTC P0171 / P0174.

Regarding the Toyota Prius heat storage tank check the following:

  • Make sure that the cooling systems for the engine and hybrid system are full and properly bled.
  • Make sure there are no kinks in the hoses going to and from the tank.
  • Check that the electric water pump is working properly.

Other Manufacturer Specific Definitions for P1151

Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 2 (Ford)
Long term fuel trim below lean limit (Volkswagen)
Long term fuel trim 1, Bank 1 - below lean limit (Audi)
Misfire Cylinder 5 With Fuel Cut-Off (BMW)
Fuel Rail Pressure Malfunction Maximum Positive Deviation (Chrysler)
Fuel Rail Pressure Malfunction Maximum Positive Deviation (Dodge)
Electronic Throttle System Main Accel Position Sensor 2 Malfunction (Hyundai)
Fuel Rail Pressure Malfunction Maximum Positive Deviation (Jeep)
Accelerator pedal position (APP) sensor – circuit malfunction (Kia)
Heated oxygen sensor (H02S), upstream, bank 2 – mixture too lean (Land Rover)
Heated oxygen sensor (H02S) 1, bank 2 -not switching, fuel trim (FT) weak mixture (Lincoln)
Heated oxygen sensor (H02S) – LH or front bank -front (6 cyl) – lean mixture indicated (Mazda)
Heated oxygen sensor (H02S) 1, bank 2 not switching, fuel trim (FT) weak mixture (Mercury)
O2 Sensor Heater Circuit Signal Intermittent Bank 2 Sensor 1 (Mini)
Heated oxygen sensor (H02S) 2 -voltage high (Saab)
Heated oxygen sensor (H02S) 2 – voltage high (Subaru)
Heated oxygen sensor (H02S) 1, bank 2 – output voltage high (Suzuki)
Heated oxygen sensor (H02S) 1, bank 2 – signal malfunction (Volvo)

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