Decat and Second Lambda Off: P0420/P0430 on MED17.5.5, ME7.5 and MEVD17 Petrol ECUs

A Golf VI GTI on MED17.5.5 comes in with a 200-cell sports downpipe fitted by the previous owner, P0420 stored, and the lamp lit. The owner says it’s been like that for a year. On a MED17 the code doesn’t hurt anything by itself, but the lamp hides every other fault, and the car will fail the OBD part of inspection with a stored catalyst code.

What P0420 and P0430 actually measure

The rear lambda sensor (sensor 2) sits after the catalyst. With a healthy cat the oxygen storage smooths out the front sensor’s oscillation, so the rear signal is slow and flat. The ECU runs a test at steady cruise, forces a small rich/lean swing and compares the rear sensor’s response with the front. If the rear follows the front too closely, oxygen storage is gone and P0420 (bank 1) or P0430 (bank 2) is stored. On VAG the same test also uses an ageing monitor that measures the delay between the two signals. Nothing else in the strategy depends on the rear sensor except fuel trim on a few older units (ME7.5 uses sensor 2 for a slow trim correction on most versions).

Options, in order of trouble

1. Replace the cat with a genuine or a Euro-approved aftermarket unit. No file work. The only legal option for a road car in most EU states.
2. Sports cat, 100 or 200 cells, with a type-approval number (ABE in Germany, homologated elsewhere). Some pass the monitor as delivered, especially 200-cell units on turbo cars. Many 100-cell cats fail P0420 after a few thousand km as the coating ages. On those a file with the cat monitor disabled is the usual fix, and the car is still legal in states that accept the ABE.
3. Spacer, ‘defouler’ or a mini cat in the rear sensor bung. Cheap, works for a while on some cars, fails on MED17 and MEVD17 because the test looks at signal timing, not just level.
4. Full decat with a rear lambda off / cat off file. Only for track or export cars.

What the file changes

A proper cat off on a petrol ECU disables the catalyst efficiency monitor (P0420/P0430), the rear sensor ageing and dynamic tests, and the rear sensor circuit and heater checks (P0136 to P0141, P0036 to P0038 on bank 1, plus the bank 2 equivalents on V6 and V8), and removes the rear sensor’s contribution to any trim. The front sensor and fuel control are untouched. The sensor itself can be removed and the bung plugged, or left in place. Readiness for the catalyst monitor is set complete so the OBD check doesn’t show ‘not ready’.

What a quick DTC-only edit misses: the rear sensor heater code (P0141) keeps returning if the sensor is unplugged, and on ME7.5 the slow trim goes wrong without the rear signal, giving a lean idle after a few weeks.

Petrol vs diesel

On diesels the ‘cat’ is the DOC in front of the DPF. Its monitor uses temperature sensors, not lambda: the ECU checks the temperature rise across the DOC during a regen (P0420 exists on EDC17 diesels too, and P2080/P2084 for the sensors). Removing the DOC without the DPF makes no sense, and with the DPF it’s covered by a DPF off file. A diesel ‘lambda off’ is a separate request: the wideband sensor on EDC16 and EDC17 feeds the smoke limit and EGR correction, and disabling it is only done when the sensor fails and the owner won’t replace it.

Inspection and legal

Petrol cars registered after 1993 are tested for CO and lambda at idle and at 2,500 to 3,000 rpm in most EU states (Germany AU, UK MOT, Netherlands APK). Without a working cat, CO at high idle sits far above the 0.2 to 0.3 % limit and the car fails no matter what the ECU says. So the file solves the lamp and the OBD readiness, not the tailpipe. A homologated sports cat is the only version of this job that passes both. We’ll say this on the order and won’t argue about it afterwards.

Which ECUs

VAG MED17.5.5 (VW GOLF MED17.5.5 03C997016 8642 1037514973, VW PASSAT MED17.5.5 03C997017 2922 1037521857), ME7.5 on 1.8T and 2.0 FSI, MED9.1, MED17.1 and MED17.5 on Audi, BMW MEVD17.2 and MSD80/81, Mini MED17.2, PSA MEV17.4, Hyundai/Kia M7.9.8 and SIM2K, Fiat IAW 5SF and 8GMF. On MED17 and MEVD17 the file needs the checksum corrected and, on BMW, the DME read in bench or boot with KTAG or Flex; ME7.5 and MED9.1 are OBD jobs with KESS V2 or MPPS.

Send the full read, whether the rear sensor stays fitted, and which cat is on the car through the request form. Model pages: MED17.5.5, ME7.5, MEVD17.2.