E133 intermittent lockout:

The intractable intermittent E133 error code on Potterton Gold, Baxi Platinum, Potterton Promax, Main Eco, Baxi Duo-Tec

Baxi make this design of boiler and sell it in huge numbers under a variety of different names and labels. If yours is having E133 lockout errors, you probably have  one of the following which are all broadly the same boiler inside the case, except for minor details:

Baxi Platinum

Potterton Gold

Potterton Promax

Main Combi Eco

Baxi Duo-Tec

More accurately, the boiler displays “E1” for a second, then the display switches to “33” for a second (as the display has only two characters), and these two alternate ad infinitum giving us “E133”.

E133 is a ‘catch-all’ fault code telling us the boiler failed to light or went out whilst running. This is not helpful as we knew that already! In more detail, it means the electronic flame monitoring process  failed to detect a flame so turned the boiler OFF and put it into a safe condition. This does not however, necessarily mean the flame really went out or failed to light. It is just as probable the flame detection process itself failed or went wrong somehow, and the boiler was actually correctly alight at the point of locking out. 

E133 is reasonably easy to trace and fix if the boiler refuses to relight after reset, but what usually happens and drives people to the internet to research it, is the boiler resets perfectly well and runs for an hour or a day or a week, then randomly locks out again for no obvious reason. And the error code only tells us what happened, not why.

The boiler manual gives us this short list of reasons for E133:

Interruption Of Gas Supply 

Flame Failure

The manual is silent on how to deal with this when it is intermittent so here is my own (rather longer) checklist of things to investigate:

1 ) Distorted ignition electrode

The electrode rods change shape (for no obvious reason) so the spark gap changes from the required 4mm to either a really big gap or a tiny small gap, and the spark no longer occurs in quite the right place and fails to reliably ignite the gas. I suspect this can happen if the combustion settings drift out of adjustment and the flame burns too hot. Fit a new electrode and check the combustion settings on high and low fire. 

2 ) Dirty or mis-shapen rectification electrode

The flame rectification (ionisation) electrode gets dirty with age and fails to accurately detect ionisation in the  burning gas in the space between itself and the burner surface. So the PCB thinks the flame has gone out even when it hasn’t. Fit a new electrode to rule this out. Or carefully clean the existing electrode. 

3 ) Combustion settings drifting away from correct values

Incorrect combustion settings can cause ignition to fail or the flame to go out. A correctly calibrated combustion analyser is needed to check the constituents of the combustion gases and adjust the gas valve if necessary. Poor combustion can be the cause of distorted electrodes as mentioned in 1 ) and 2 ) above. 

4 ) Ignition lead failing

The spark ignition lead insulation begins to fail and the spark can jump straight through the insulation to any close metal part of the boiler, instead of jumping across the electrode spark gap. Replacing the lead might fix this but I don’t understand why it happens in the first place. This is a problem on lots of boilers (not just the Baxi Platinum). If the spark gap is too big then a high enough voltage to break through the lead insulation will be developed by the ignition module but it can happen even with the correct spark gap. A new ignition lead often helps. 

5 ) Condensate trap or drain blocked 

I see this mentioned repeatedly as a possible cause of E133 but unlike many boilers, this boiler has no system for detecting a blocked condensate drain (unlike many other makes and models). If the drain fully blocks the boiler will continue to run and eventually the combustion chamber will fill with condensate the boiler will go out. But I think the combustion chamber door seal would leak condensate as a tell-tale so this happening would be obvious to anyone looking inside the boiler.

6 ) Ignition module failure

The ignition module (spark generator) can fail but I don’t think intermittent failure is very likely. The module is cheap to replace as a speculative repair to rule it out though, and might fix an intermittent E133.

7 ) Gas valve intermittent failure

Again this is near-impossible to devise a test for, so swapping the gas valve as a speculative repair is the only way to rule this out I can think of. The mechanism of intermittent gas valve failure is a break in the ultra-fine wire of the solenoid coil touches most of the time and a circuit is maintained, until after a long time running the rising heat in the coil causes expansion to break the circuit, the gas valve closes and the boiler goes out. After ten minutes the coil cools, shrinks again slightly and the circuit id re-made, and the boiler works again until next time it happens. 

8 ) PCB failure

Grasping at straws, this one. I don’t know how intermittent E133 can be caused by a failing PCB but I’ve heard claims of a new PCB fixing it. 

Summary:

Intermittent locking out can be fiendishly difficult to fix and (understandable) impatience from the user can lead to a relatively young boiler being scrapped and replaced as the only way to get a quick resolution. Other condensing boilers from other manufacturers also have similar problems with intermittent locking out, with equally unhelpful documentation which simply ignores the problem. On days when I’m feeling particularly cynical I sometimes wonder if this could possibly be deliberate, with users being driven to buy new boilers when their boiler technician is unable to trace and fix the problem. On reflection though, surely not!