I’m having problems with my Toyota RAV4. I bought the car 6 months ago with a lambda sensor fault. By time I got it home, had lambda bank 2, sensor 2 and Mass airflow sensor faults. Changed both, okay for 300 miles then had lambda sensor bank 2, sensor 1. Changed this and was fine for 5 miles then back to original lambda sensor fault.

It’s failed MOT by emissions. CO @ 6.81%. should be max 0.3% HC @ 459ppm should be max 200ppm Lambda @ 0.82 should be 0.97-1.03

The CO is x20 what it should be… I’ve had it in 3 garages, electrical specialists. Had wiring harness checked, put as close to OEM lambda sensors in, checked for vacuum leaks. Just going round in circles with the fault codes and changing sensors

What do I do?

  • If you changed a sensor and a sensor fault is again reported, either the new sensor was duff (unlikely if new) or the detection mechanism is faulty. Who shall guard the guards? So perhaps the ECU is at fault. On aging electrical systems it is often worth disturbing every electrical connector, to ensure good electrical contacts are made, as corrosion can affect them over time. Perhaps just wiggle every connector to scrape the mating contacts. Apr 3, 2023 at 18:44
  • I’ve had the ECU checked. All wiring and connections checked. Replaced the lambda sensors twice to ensure they weren’t duff
    – LEP
    Apr 3, 2023 at 18:57
  • If it has failed its MOT test the CO levels are checked independently of the vehicle's own sensors. Apr 3, 2023 at 19:54
  • @WeatherVane The Lambda sensor can be flagged with an error for continuously reporting a too rich mixture. If the lambda sensor reports a rich mixture, the ECU will attempt to inject less fuel to compensate. If the ECU reaches its maximum configured fueling adjustment, it will presume something is wrong with the sensor and flag a fault. There is likely something else causing the over fueling.
    – HandyHowie
    Apr 4, 2023 at 7:11

2 Answers 2


Given the age of the car, check the compression on all cylinders before trying anything else. You don't want to spend money for repairs on a car with a blown engine. While you are at it, be sure to check the spark plugs.

The combination of improper MAF reading and unburned fuel points directly at poor compression (and thus weak intake manifold vacuum).


Following what you have done, the next thing I would check would be the fuel pressure, by adding a fuel pressure gauge to the injector rail supply.

If the fuel pressure is higher than it should be, then the injectors will inject more fuel than the ECU is intending.

You can also use this gauge to check that the injectors are not leaking when closed, which could also cause an issue. After running the engine and shutting it down, the fuel pressure should stay constant for some time. If the fuel pressure drops quickly, it is a sign that an injector is leaking.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .