CAT/LAMBDA Off — disabling catalyst monitoring and the rear O2 sensor

CAT/LAMBDA OFF switches off the second lambda sensor, the one behind the catalyst, in the software of a petrol ECU, together with the catalyst monitoring based on it. The file removes catalyst efficiency faults (P0420, P0430) and second-sensor faults, while the first sensor and mixture control keep working as before. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

How the catalyst and the two sensors work together

A three-way catalyst does three jobs at once: it burns off CO and hydrocarbons and reduces nitrogen oxides to nitrogen. It only manages this in a narrow window around λ = 1. Slightly lean and NOx stops being reduced, slightly rich and CO and hydrocarbons slip through. The active metals are platinum, palladium and rhodium, and cerium oxide in the coating acts as an oxygen buffer: it stores oxygen when the mixture is lean and releases it when it is rich.

Keeping the mixture in that window is the job of the first sensor, mounted ahead of the catalyst. The ECU corrects injection from its signal all the time, so the mixture swings around λ = 1, slightly rich, then slightly lean. The second sensor behind the catalyst is there to keep watch: the ECU compares its signal with the first one and decides whether the catalyst is still alive. On many engines the second sensor is also used to fine-tune the first control loop.

How the ECU checks the catalyst and where P0420 comes from

A healthy catalyst smooths out the mixture swings with its oxygen store. That is why in steady driving the second sensor's signal barely moves, while the first one swings several times a second. Once the coating is worn, there is not enough stored oxygen and the second sensor starts copying the first. From these signals the ECU works out a catalyst efficiency figure, and if it falls below the threshold it stores P0420 (bank 1) or P0430 (bank 2 on V engines) and turns on the Check Engine light. The ECU does not run this check all the time, only with a warm engine under steady conditions, so after an exhaust change the fault may not show up on the first drive.

The second sensor sees the same picture when there is no catalyst at all, or when the factory unit has been replaced with a sports catalyst that stores less oxygen. To the ECU it makes no difference: the signals look alike, so the catalyst is not working. In the EU, on-board diagnostics have required petrol cars to monitor the catalyst with a second sensor since 2000, so without a software change, removing the catalyst almost always ends with a Check Engine light.

Tuners usually call this a decat file or a rear O2 delete. Only the second sensor is switched off: the first one is still needed to control the mixture. Catalysts themselves have been mandatory on every new petrol car in the EU since 1993, the Euro 1 days.

Factory, sports catalyst or straight pipe: what the ECU sees

Simply unscrewing the second sensor without a file does not work either: the ECU immediately sees an open sensor and heater circuit and logs second-sensor faults. Without a catalyst, they arrive together with P0420.

When CAT/LAMBDA OFF is the answer, and when to repair first

What CAT/LAMBDA OFF changes in the software

Clearing P0420 with a scan tool is pointless: on the next drive the ECU compares the signals again and stores the same fault. So the logic in the file itself is changed.

The first sensor is not disabled: without it the ECU cannot hold the mixture around λ = 1. The rest of the calibration is unchanged too, so the file adds no power by itself. If the new exhaust calls for more power, add Stage 2. The same file can also take OPF/GPF Off, if the car has a petrol particulate filter, and Pop & Bangs. Diesels use their lambda sensor differently, and there is a separate LAMBDA OFF option for them.

Faults that go away together with catalyst and second-sensor monitoring:

Codes per SAE J2012. Bank 1 is the side of the engine with cylinder 1. Manufacturers may also use their own codes for the same faults. First-sensor faults are not on this list.

What to do with the exhaust and sensors

• The first sensor has to be healthy: the ECU holds the mixture with it, and the file does not remove its faults.
• The second sensor can stay in place after flashing, or be removed with its port blanked off: the ECU no longer uses or checks its signal.
• V engines: there are two catalysts and two second sensors, one per bank, and both are disabled.
• Pop & Bangs with a live catalyst. Fuel burning in the exhaust overheats the catalyst just like misfires do. Keep that in mind if the catalyst stays.

How to order the file

1. Read the original file via OBD or on the bench with the tool you use. We need an unmodified stock file.
2. Upload it to the portal and select CAT/LAMBDA OFF. You can add a Stage, OPF/GPF OFF or Pop & Bangs at the same time.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file, clear the faults and check after a drive that P0420 and the second-sensor faults have not come back.

The option is available for petrol ECUs including Bosch ME, MED, MEVD and MG, Continental Simos, SIM and EMS, Delco and Kefico, and the full list by make is below. If your ECU is not there, send the file with a request: we will check the software and tell you whether there is a solution.

Removing or disabling the catalyst increases CO, hydrocarbon and NOx emissions and changes the vehicle's emissions class. In the EU, the UK, the US and a number of other countries this is prohibited on public roads and may cause an inspection failure. The service is intended for tuners and is used where the law allows it: motorsport, off-road, export, vehicles not used on public roads. Responsibility for using the file lies with the vehicle owner.